Chemistry 10th

7 CHAPTER 01 CHEMICAL REACTIONS AND EQUATIONS CHAPTER AT A GLANCE 1. The following observations help us to determine whether a chemical reaction has taken place: (a) Change in state (b) Change in colour (c) Evolution of a gas (d) Change in temperature. 2. Law of conservation of mass: It states that, “Mass can neither be created nor destroyed in a chemical reaction, i.e., the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.” 3. The symbolic representation of a true chemical change or reaction is called a chemical equation. It is used to describe a chemical reaction in a shorter form. 4. Word equation for a reaction: Magnesium + Oxygen ¾® Magnesium oxide (Reactants) (Products) The substances that undergo chemical change in the reaction are called reactants, e.g., magnesium and oxygen in the above reaction. The new substance formed during the reaction is the product, e.g., magnesium oxide in the above reaction. 5. Chemical equation can be made more concise and useful by using chemical formulae instead of words. A chemical equation represents a chemical reaction. The chemical equation for the above word equation can be written as: 2Mg(s) + O2( g ) ¾® 2MgO(s) Magnesium Oxygen Magnesium oxide 6. Types of chemical reactions: 1. Combination reaction: A chemical reaction in which two or more substances (elements or compounds) combine to form a single substance is called a combination reaction. The combination reactions are of three types: (i) Combination between two elements: e.g., 2H2(g) + O2(g) → 2H2O(l) N2(g) + 3H2(g) → 2NH3(g) C(s) + O2(g) → CO2(g) The combination reactions (as shown above) in which a new compound is formed from its constituent elements are also called synthesis reactions. (ii) Combination between two compounds: e.g., CaO(s) + H2O(l ) → Ca(OH)2(aq) + Heat (Quicklime) (Slaked lime) Note: This is an example of exothermic reaction. NH3( g) + HCl(g) → NH4Cl(s) Ammonia Hydrogen Ammonium chloride chloride

8 CHEMICAL REACTIONS AND EQUATIONS Note: The reaction (between NH3 and HCl) is used as a test to detect the presence of NH3. During this reaction, white fumes (due to the formation of NH4Cl) can be seen. (iii) Combination between an element and a compound: e.g., 2CO(g) + O2(g) → 2CO2(g) 2NO(g) + O2(g) → 2NO2(g) 2. Decomposition reaction: A chemical reaction in which a single compound decomposes into two or more simpler substances (elements or compounds) is called a decomposition reaction. The decomposition reactions are endothermic reactions as they require energy in the form of heat, light or electricity. Due to this fact, decomposition reactions are of three types: (i) Thermal decomposition: It is a decomposition reaction brought about by heat. e.g., CaCO3(s) ¾H¾ea¾t® CaO(s) + CO2(g) Limestone Quicklime 2Pb(NO3 )2(s) ¾H¾ea¾t® 2PbO(s) + 4NO2( g) + O2( g) Lead nitrate Lead oxide Nitrogen Oxygen dioxide 2FeSO4(s) ¾H¾ea¾t® Fe2O3(s) + SO2( g) + SO3(g) Ferrous sulphate Ferric oxide (Smell of burning (Light green) (Dark brown) sulphur) (ii) Photolysis: It is a decomposition reaction brought about by light. e.g., 2AgCl(s) ¾S¾un¾ligh¾t® 2Ag(s) + Cl2(g) Silver chloride turns grey in sunlight. 2AgBr(s) ¾S¾un¾ligh¾t® 2Ag(s) + Br2(g) Note: The above reactions are used in black and white photography. (iii) Electrolysis: It is a decomposition reaction brought about by electricity. 2NaCl(l) ¾E¾lec¾tric¾cur¾rent¾® 2Na(s) + Cl2( g) Sodium chloride Sodium Chlorine (At cathode) (At anode) 3. Displacement reaction: A reaction in which a more active element displaces less active element from its compound present in dissolved state is called a displacement reaction. Note: 1. A more electropositive metal displaces a less electropositive metal from its aqueous solution. 2. A more electronegative non-metal displaces a less electronegative non-metal from its aqueous solution. Examples: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) Zinc Copper sulphate Zinc sulphate Copper (Zinc displaces copper from CuSO4 solution) Cu(s) + 2AgNO3(aq) → Cu(NO3 )2(aq) + 2Ag(s) Copper Silver nitrate Copper nitrate Silver (Cu displaces Ag from AgNO3 solution) Fe(s) + CuSO4 ( aq ) → FeSO4 ( aq ) + Cu(s) Iron Copper sulphate Iron(II) sulphate Copper (Fe displaces Cu from CuSO4 solution)

CHEMICAL REACTIONS AND EQUATIONS 9 Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s) Lead Copper chloride Lead chloride Copper Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(l) Sodium bromide Chlorine Sodium chloride Bromine (Cl2 displaces Br2 from its salt solution) 4. Double displacement reaction: A chemical reaction in which there is an exchange of ions between the reactants to give new substances is called a double displacement reaction. The two reactants are generally water soluble. Such double displacement reactions take place with the: (i) formation of a precipitate, (ii) evolution of a gas, or (iii) change in colour. e.g., BaCl2(aq) + Na2SO4(aq) → BaSO4(s) ↓ + 2NaCl(aq) Sodium chloride Barium chloride Sodium sulphate Barium sulphate (White ppt.) NaCl(aq) + AgNO3(aq) → AgCl(s) ↓ + NaNO3(aq) Silver chloride Sodium chloride Silver nitrate (White ppt.) Sodium nitrate Na2SO4(aq) + BaCl2(aq) → 2NaCl(aq) + BaSO4(s) ↓ Sodium sulphate Barium chloride Sodium chloride Barium sulphate (white ppt.) 5. Energy changes during chemical reactions: Energy changes occur during chemical reactions because different amounts of energies are involved in breaking a bonds in reactants and making of bonds in products. Depending upon whether heat is evolved or absorbed during a reaction, the reaction can be exothermic or endothermic. (a) Exothermic reaction: A chemical reaction in which heat energy is given out is called an exothermic reaction. Burning of natural gas, respiration, formation of compost from vegetable matter are also exothermic processes. e.g., C(s) + O2(g) → CO2(g) + 94.00 kcal 2H2(g) + O2(g) → 2H2O(l) + 136.0 kcal (b) Endothermic reaction: A chemical reaction in which heat energy is absorbed is called an endothermic reaction. e.g., C(s) + 2S(s) → CS2(l) – 22 kcal C(s) + H2O(g) ¾1¾100¾°C¾® CO(s) + H2(s) – 31.4 kcal 6. Oxidation and Reduction reaction: (a) Oxidation: It is a chemical process in which a substance gains oxygen or loses hydrogen. Addition of oxygen: Element + Oxygen → Oxide e.g., C(s) + O2(g) → CO2(g) 2Mg(s) + O2(g) → 2MgO(s) In the above reactions: (i) Carbon is oxidised to CO2. (ii) M agnesium is oxidised to MgO by the gain of oxygen.

10 CHEMICAL REACTIONS AND EQUATIONS Removal of hydrogen: H2S(g) + Cl2(g) → 2HCl(g) + S(s) In this reaction, hydrogen sulphide is oxidised to sulphur by the loss of hydrogen. (b) Reduction: It is a chemical process in which a substance gains hydrogen or loses oxygen. Addition of hydrogen: 2NH3(g) + 3Cl2(g) → N2(g) + 6HCl(g) In this reaction, chlorine is reduced to hydrogen chloride by the gain of hydrogen. Removal of oxygen: Oxide + Element → Metal ZnO(s) + C(s) → Zn(s) + CO(g) Redox reactions: Consider the following reaction: In this reaction: (i) The copper(II) oxide is losing oxygen and being reduced. (ii) The hydrogen is gaining oxygen and is being oxidised. Such reactions in which one reactant gets oxidised and the other gets reduced are called oxidation-reduction or redox reactions. Some other examples of redox reactions are: MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 Fe2O3 + 3CO → 2Fe + 3CO2 Oxidising agent: The substance which can bring about oxidation of other substances is called an oxidising agent. Or A substance which causes addition of oxygen or removal of hydrogen from other substances is called an oxidising agent. e.g., S(s) + O2(g) → SO2(g) (Oxygen is the oxidising agent) H2S(aq) + Cl2(g) → 2HCl(aq) + S(s) (Chlorine is the oxidising agent) Reducing agent: The substance which can bring about reduction of other substances is called a reducing agent. Or A substance which causes addition of hydrogen or removal of oxygen from other substances is called a reducing agent. e.g., C(s) + ZnO(s) → CO(g) + Zn(s) Carbon is the reducing agent as it is removing oxygen from ZnO. H2S(g) + Cl2(g) → 2HCl(g) + S(s) H2S is the reducing agent as it is adding hydrogen to chlorine.

CHEMICAL REACTIONS AND EQUATIONS 11 7. Electronic concept of oxidation and reduction: Oxidation: In the electronic concept, oxidation is defined as a process in which an atom or an ion loses electrons. e.g., (i) Formation of Zn2+ ion from Zn. Zn → Zn2+ + 2e– (ii) Formation of ferric (Fe3+) ion from ferrous (Fe2+) ion. Fe2+ → Fe3+ + e– Reduction: In the electronic concept, reduction is defined as a process in which an atom or an ion gains electron(s). e.g., (i) Formation of Zn from Zn2+ Zn2+ + 2e– → Zn (ii) Formation of Cl– from Cl2 Cl2 + 2e– → 2Cl– Oxidising agent: In the electronic concept, the chemical species which can remove electrons from some other species is called an oxidising agent. The oxidising agent gets reduced during a redox reaction by gaining electrons. e.g., Sn4+ + 2e– → Sn2+ (Sn4+ is an oxidising agent) Tin(IV) ion Tin(II) ion F2 + 2e– → 2F– ion (F2 is an oxidising agent) Fluorine Fluoride Reducing agent: In the electronic concept, the chemical species which can lose electrons to some other species is called a reducing agent. The reducing agent gets oxidised during a redox reaction by losing electrons. Fe2+ → Fe3+ + e– (Fe2+ is a reducing agent) Iron(II) ion Iron(III) ion 2Cl– → Cl2 + 2e– (Cl– is a reducing agent) Chloride ion Chlorine 8. Corrosion: It is the slow eating up of the metals by the action of air and moisture on their surfaces. During corrosion, the pure metal changes to its original form, i.e., mineral or ore. It is also called weeping of metals. It is an undesirable property and has to be prevented. Some examples of corrosion are: (i) Formation of oxide layer on the surface of aluminium. (ii) Tarnishing of silver metal on exposure to hydrogen sulphide. (iii) Formation of reddish brown layer on the surface of iron articles, etc. Corrosion in case of iron is called rusting. Chemically, rust is hydrated ferric oxide, Fe2O3.xH2O. Note: Gold and silver do not corrode in moist air because these are noble metals and are not attacked by air and moisture. Conditions necessary for rusting (corrosion of iron) are: (i) Exposed metal surface. (ii) The metal surface must come in contact with air or oxygen. (iii) The air must contain moisture. Various methods used to prevent rusting: Iron can be prevented from rusting by coating its surface with an appropriate material so that its surface does not come in contact with air. The various methods used to prevent rusting are : (A) Non-metallic coating 1. By painting with enamel paints. e.g., iron doors, windows, etc.

12 CHEMICAL REACTIONS AND EQUATIONS 2. By enamelling. e.g., cooking stoves, refrigerators, etc. 3. By coating with red lead oxide paint or tar. e.g., lower parts of ship or bridge. 4. By oils and greases. e.g., moving parts of a machinery such as vehicles, coolers, etc. (B) Metallic coating 1. By galvanising. e.g., roof sheds, buckets, tubs, etc. 2. By tin plating. e.g., tinned boxes for fruits, oil, meat, etc. 3. By coating with nickel or chromium electrolytically. e.g., bicycle handles, rims, bumpers of cars, etc. 4. By forming alloys. e.g., surgical instruments, utensils, etc. Sometimes the corrosion is an advantage because it prevents the metal underneath from further damage. For example, on exposure to the air, the aluminium surface gets coated with a thin sticking oxide layer (Al2O3) which protects the metal underneath from further corrosion and damage. This oxide layer is stable even at high temperatures. Due to this reason, aluminium utensils are used for cooking purposes. 9. Rancidity: When oils and fats or foods containing oils and fats are exposed to air or oxygen, they get oxidised due to which the food becomes stale and its colour and smell changes. It is called rancidity. The rancidity of fatty foods can be prevented by adding antioxidants and flushing the food containers with nitrogen to prevent food from oxidation. At home oxidation process of food can be slowed down by keeping it in the refrigerator or deep freezers. NOTES

CHEMICAL REACTIONS AND EQUATIONS 13 ASSIGNMENT (In Class Discussion) Q.1. A chemical reaction is represented by writing a ________. Options: (a) Chemical name (b) Chemical formulae (c) Chemical equation (d) Name of the catalyst Q.2. A chemical reaction involves__________. Options: (a) Only breaking of bonds (b) Only formation of bonds (c) Both breaking and formation of bonds (d) None of these Q.3. A balanced chemical equation always obeys___________. Options: (a) Law of conservation of Mass (b) Law of thermal equilibrium (c) Law of conservation of energy (d) All of the above Q.4. H2 + O2 ® H2O, what are the coefficients used to balance this equation? Options: (a) 2, 2, 2 (b) 0, 2, 2 (c) 2, 1, 2 (d) 2, 0, 2 Q.5. The reaction CaCO3 ¾¾∆ ®CaO + CO2 is a _______. Options: (a) Displacement reaction (b) Combination reaction (c) Decomposition reaction (d) Double displacement reaction Q.6. What is observed when a solution of potassium iodide (KI) is added to lead nitrate Pb(NO3)2? Options: (a) No reaction takes place (b) Yellow precipitate of lead iodide is formed (c) White precipitate of lead iodide is formed (d) Lead iodide is soluble in water Q.7. “We need to balance an unbalanced skeletal chemical equation”. Justify the statement. Q.8. Balance the equation stepwise: H2S2O7 (l) + H2O ® H2SO4 Q.9. Explain the terms reactant and product by giving examples. Q.10. What is the importance of a chemical equation? Q.11. State the type of chemical reactions and chemical equations that take place in the following: (i) Magnesium wire is burnt in air. (ii) Electric current is passed through water (iii) Ammonia and hydrogen chloride gases are mixed. Q.12. What happens when current is passed through acidulated water? Q.13. Differentiate between displacement and double displacement reaction. Q.14. What is a word equation and how it is written? Q.15. What do you understand by precipitation reaction? Explain with examples.

14 CHEMICAL REACTIONS AND EQUATIONS Q.16. In a redox reaction, oxidation and reduction reactions take place ________. Options: (a) Separately (b) Together (c) First reduction, then oxidation (d) First oxidation, then reduction Q.17. Rancidity is an example of ____________. Options: (a) reduction (b) oxidation (c) combination (d) displacement Q.18. What are the necessary conditions for rusting? Options: (a) Air (b) Water (c) Nitrogen (d) Both (a) and (b) Q.19. Corrosion of metals involves Options: (a) Physical change (b) Chemical change (c) Periodic change (d) Reversible change Q.20. The oxidation reaction is one in which _______. Options: (a) Oxygen is lost (b) Oxygen is added (c) Hydrogen is lost (d) Both b and c Q.21. The reduction reaction is one in which ______ Options: (a) Oxygen is lost (b) Hydrogen is added (c) Hydrogen is lost (d) Both a and b Q.22. Identify oxidant and reductant respectively in the below reaction. Options: (a) Cl2, H2S (b) S, HCl (c) S, Cl2 (d) HCl, H2S Q.23. In this equation CuO(s) + H2(g) → Cu(s) + H2O(g) Which substance gets reduced? Options: (a) CuO (b) Cu (c) H2O (d) H2 Q.24. Explain two ways by which food industries prevent rancidity. Q.25. What is corrosion? Do gold ornaments corrode? Justify. Q.26. Differentiate between oxidation and reduction. Q.27. Identify from the following reactions the reactants that undergo oxidation and reduction. (i) Fe + S ® FeS (ii) 2Ag2O ® 4Ag + O2 (iii) 2Mg + O2 ® 2MgO

CHEMICAL REACTIONS AND EQUATIONS 15 Q.28. Identify the substance oxidised and the substance get reduced in the following reactions. (i) 2PbO + C ® 2Pb + CO2 (ii) MnO2 + 4HCl ® MnCl2 + 2H2O + Cl2 Q.29. You might have noted that when copper powder is heated in a china dish, the surface of copper powder becomes coated with a black colour substance. (i) How has this black coloured substance formed? (ii) What is that black substance? (iii) Write the chemical equation of the reaction that takes place. Q.30. When a magnesium ribbon burns in air with a dazzling flame and forms a white ash, is magnesium oxidised or reduced. Why? NOTES

16 CHEMICAL REACTIONS AND EQUATIONS ASSERTIONS AND REASONING TYPE QUESTIONS (Self-Practice) I. Directions: In the following questions, two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below: (a) Both (A) and (R) are true and (R) is correct explanation of the assertion. (b) Both (A) and (R) are true but (R) is not the correct explanation of the assertion. (c) (A) is true but (R) is false. (d) (A) is false but (R) is true. Q.1. Assertion (A): Stoichiometric coefficient are the numbers which are put before compounds or elements to balance the chemical equation. Reason (R): Chemical equation is balanced to justify the law of conservation of mass. Q.2. Assertion (A): When zinc rod is dipped into aqueous solution of copper sulphate, the colour of the solution changes. Reason (R): Zinc being more reactive displaces less reactive. Copper from its aqueous solution resulting in the formation of zinc sulphate which is colourless. Q.3. Assertion (A): Heat is required for the decomposition of lead nitrate. Reason (R): Decomposition reactions are endothermic. Q.4. Assertion (A): During double displacement reactions, precipitates are usually formed. Reason (R): Precipitation reactions produce insoluble salt. Q.5. Assertion (A): Reactions also involve the gain or loss of oxygen or hydrogen by substances. Reason (R): Oxidation is the gain of oxygen or loss of hydrogen while reduction is the loss of oxygen or gain of hydrogen. II. Directions: In the following questions, two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below: (a) Both (A) and (R) are true and (R) is correct explanation of the assertion. (b) Both (A) and (R) are true but (R) is not the correct explanation of the assertion. (c) (A) is true but (R) is false. (d) (A) is false but (R) is true. Q.1. Assertion (A): Photosynthesis is an endothermic reaction. Reason (R): Energy from sunlight is absorbed in photosynthesis. Q.2. Assertion (A): All decomposition reactions are generally endothermic reactions. Reason (R): All decomposition reactions generally occur with the adsorption of heat or light energy. Q.3. Assertion (A): Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) is a double displacement reaction. Reason (R): Two types of products are formed. Q.4. Assertion (A) : 2Mg + O2 ¾¾Hea¾t ® 2MgO is a redox reaction. Reason (R): Mg is oxidised and O2 is reduced. Q.5. Assertion (A): Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq) It is a double displacement reaction. Reason (R): Two types of products are formed. Q.6. Assertion (A): Reduction occurs with the gain of hydrogen atoms. Reason (R): Oxidation occurs with the loss of hydrogen atom.

CHEMICAL REACTIONS AND EQUATIONS 17 QUESTION BANK Short Answer Type Questions (3 Marks) Q.1. Explain giving chemical equation any two uses of chemical decomposition reaction in industry. Q.2. Name the type of reaction for the following: (i) Vegetable matter changing into compost. (ii) Burning of natural gas. (iii) Silver items turning black when left exposed to air. (iv) Change in colour of copper sulphate for blue and green on dipping a nail in it. Q.3. (i) When a metal ‘X’ is added to salt solution of a metal ‘Y’ , following chemical reaction takes place : Metal X + Salt solution of ‘Y’ → Salt solution of ‘X’ + Metal ‘Y’. (ii) Mention the inference you draw regarding the reactivity of metal ‘X’ and ‘Y’ and also about the type of reaction. State the reason of your conclusions. Q.4. When potassium iodide solution is added to a solution of lead (II) nitrate in a test tube, a precipitate is formed. (i) What is the colour of this precipitate? (ii) Name the compound precipitated. (iii) Write a balanced chemical equation for this reaction. (iv) What type of reaction is this? Q.5. An aluminium can was used to store ferrous sulphate solution. It is observed that in few days holes appeared in the can. Explain the observation and write chemical equation to support your answer. Q.6. (a) Why is combustion reaction an oxidation reaction? (b) How will you test whether the gas evolved in a reaction is hydrogen? Q.7. What is the colour of FeSO4. 7H2O crystals? How does this colour change upon heating? Give balanced chemical equation for the change. Q.8. Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity. Q.9. State what happens when: (a) Hydrated copper sulphate is heated (b) Gypsum is heated at 373 K. (c) Chlorine gas is passed through dry slaked lime. Also write the chemical equation in each case. Q .10. Write the chemical equation of the reaction with an example each in which the following changes have taken place: (i) change in colour. (ii) change in temperature. (iii) formation of precipitate. Q.11. (a) Write one example for each of the decomposition reactions carried out with the help of: (i) electricity (ii) heat (iii) sunlight Give balanced chemical equations in each case. (b) Which of the following statement is correct and why? Copper can displace silver from the solution of silver nitrate and silver can displace copper from the solution of copper sulphate.

18 CHEMICAL REACTIONS AND EQUATIONS Q.12. Observe the following diagram and answer the questions that follow: (i) Identify the gases evolved at anode and cathode. (ii) Why are the amounts of gases collected in the two test tubes are of not the same volume? (iii) What type of reaction is this? (iv) Why should we use salt water? Q.13. (a) Give an example for a combination reaction which is exothermic. (b) Identify the oxidising agent, reducing agent in the following reaction: H2S + Cl2 → 2HCl + S (c) Name the phenomenon due to which the taste and smell of oily food changes when kept for a long time in open. Suggest one method to prevent it. Q.14. A reddish-brown coloured metal, used in electrical wires, when powdered and heated strongly in an open china dish, its colour turns black. When hydrogen gas is passed over this black substance, it regains its original colour. Based on the above information, answer the following questions: (i) Name the metal and the black coloured substance formed. (ii) Write balanced chemical equations for both the reactions. Q.15. The following diagram displays a chemical reaction. Observe carefully and answer the following questions : (a) Identify the type of chemical reaction that will take place and define it. How will the colour of the salt change? (b) Write the chemical equation of the reaction that takes place. (c) Mention one commercial use of this salt. Q.16. A small amount of quicklime is added to water in a beaker. (a) Name and define the type of reaction that has taken place. (b) Write balanced chemical equation for the above reaction. Write the chemical name of product obtained. (c) State two observations that you will make in the reaction. Q.17. A metal ‘X’ acquires a green colour coating on its surface on exposure to air. (i) Identify the metal ‘X’ and name the process responsible for this change. (ii) Name and write chemical formula of the green coating formed on the metal. (iii) List two important methods to prevent the process. Q.18. 2 g of ferrous sulphate crystals are heated in a boiling tube. (i) State the colour of ferrous sulphate crystals both before heating and after heating. (ii) Name the gases produced during heating. (iii) Write the chemical equation for the reaction. Q.19. (a) In the following reaction: MnO2 + 4HC1 → MnCl2 + 2H2O + Cl2 Identify the oxidant and reductant. (b) Give reasons: (i) A ntioxidants are added to foods containing fats and oils. (ii) White silver chloride turns grey in sunlight. Q.20. What is water of crystallisation? Name and give the formula of a salt which contains water of crystallisation. Q.21. (a) Silver articles generally turn black when kept in the open for a few days. The articles when rubbed with tooth-paste again start shining. (i) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved. (ii) Name the black substance formed and give its chemical formula.

CHEMICAL REACTIONS AND EQUATIONS 19 (b) Write the formulae of (i) calcium phosphate and (ii) ammonium carbonate. Q.22. In the electrolysis of water, (i) Name the gas collected at anode and cathode. (ii) Why is the volume of gas collected at one electrode double than the other? (iii) What would happen if dil H2SO4 is not added to water? Q.23. 2 g of silver chloride is taken in a china dish and the china dish is placed in sunlight for sometime. What will be your observation in this case? Write the chemical reaction involved in the form of a balanced chemical equation. Identify the type of chemical reaction. Or Identify the type of reactions taking place in each of the following cases and write the balanced chemical equation for the reactions. (a) Zinc reacts with silver nitrate to produce zinc nitrate and silver. (b) Potassium iodide reacts with lead nitrate to produce potassium nitrate and lead iodide. Long Answer Type Questions (5 Marks) Q.1. (a) Write a balanced chemical equation for the process of photosynthesis and the conditions of the reaction giving physical states of all the substances. (b) Classify the following chemical reactions as exothermic or endothermic: (i) Electrolysis of water (ii) Burning of natural gas (iii) Decomposition of calcium carbonate. (iv) Burning of magnesium ribbon in air. Q.2. (a) Define corrosion. (b) What is corrosion of iron called? (c) How will you recognise the corrosion of silver? (d) Why corrosion of iron is a serious problem? (e) How can we prevent corrosion? Q.3. (a) Balance the chemical equation: Fe(s) + H2O(g) → Fe3O4(s) + H2(g) (b) Identify the type of reaction in the equation given below: Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + NaCl(aq) (c) You could have noted that when copper powder is heated in a china dish, the surface of copper powder becomes coated with black colour substance. (i) Why has this black coloured substance formed? (ii) What is this black substance? (iii) Write the chemical equation of the reaction that takes place. Q.4. Translate the following statements into chemicals equations and then balance them. (a) Hydrogen gas combines with nitrogen to form ammonia. (b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide. (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas. (e) Aluminium chloride reacts with ammonium hydroxide to form a gelatinous white precipitate of aluminium hydroxide and a salt of ammonium chloride.

20 CHEMICAL REACTIONS AND EQUATIONS APPLICATION BASED QUESTIONS (Self-Practice) Q.1. Write a balanced chemical equation for the reaction in which phosphorus burns in chlorine to form phosphorus pentachloride. Q.2. List two observations which you record while burning magnesium in air. Q.3. Identify the type of reactions in each of the following reactions: (i) Zn + H2SO4 → ZnSO4 + H2 (ii) CaO + H2O → Ca(OH)2 (iii) CaCO3 ¾H¾eat¾® CaO + CO2 (iv) AgNO3(aq) + NaCl (aq) → AgCl(s)↓ + NaNO3(aq) Q.4. Translate the following reactions into balanced chemical equations: (i) Manganese dioxide is heated with aluminium powder. (ii) Iron is treated with steam. Q.5. Define a chemical equation. What is an unbalanced chemical equation called? Q.6. Balance the following chemical equations: (a) KClO3 → KCl + O2 (b) Na2CO3 + HCl → NaCl + H2O + CO2 Q.7. Write balanced chemical equations for the following reactions: (i) Calcium carbonate on reaction with hydrochloric acid gives calcium chloride, water and carbon dioxide. (ii) Nitrogen reacts with hydrogen under pressure to form ammonia. (iii) Carbon disulphide burns in air to give carbon dioxide and sulphur dioxide. Q.8. Write two observations each for the following chemical reactions: (a) Dilute sulphuric acid is poured over zinc granules. (b) Potassium iodide solution is added to lead nitrate solution. (c) Lead nitrate is strongly heated in a hard glass test tube. Q.9. State three applications of a decomposition reaction. Q.10. Which two observations will be made when quicklime is added to water? Mention two uses of the product obtained. Q.11. A metal ‘P’ when exposed to the moist air for longer period of time, loses its shiny brown surface and gains a green coat. Why has this happened? Identify the metal. Write the name and chemical formula of this green coloured compound. List two ways to prevent this process. Q.12. What happens when a piece of: (i) Aluminium metal is added to dilute hydrochloric acid (ii) Zinc metal is added to copper sulphate solution (iii) Silver metal is added to copper sulphate solution Also write balanced chemical equation, if the reaction occurs. Q.13. Complete and balance the following chemical equations: (i) CaCO3 + HCl →

CHEMICAL REACTIONS AND EQUATIONS 21 (ii) Al + HCl → (iii) MnO2 + HCl → Q .14. Explain the type of reactions represented by the following equations: (i) CaO + CO2 → CaCO3 (ii) 2Na + H2O → 2NaOH + H2 (iii) Mg + CuSO4 → MgSO4 + Cu (iv) NH4NO2 → N2 + 2H2O (v) CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4 NOTES

22 CHEMICAL REACTIONS AND EQUATIONS ANALYZING, EVALUATING & CREATING TYPE QUESTIONS (Self-Practice) Q.1. Give reasons for the following: (i) Keeping food in airtight containers help in preventing rancidity. (ii) Moist air and acidic gases are not good for some metals. (iii) Chips manufacturers usually flush bags of chips with nitrogen gas. Q.2. Give reason why: (i) Sodium acts as an reducing agent while chlorine acts as a oxidising agent in the following reaction: 2Na(s) + Cl2(g) → 2NaCl(s) (ii) White coloured silver chloride turns grey when kept in sunlight. (iii) Why is decomposition reaction called the opposite of combination reaction? NOTES

CHEMICAL REACTIONS AND EQUATIONS 23 CHAPTER TEST (Self-Practice) Time: 1½ Hour Maximum Marks: 50 Answer The Following Questions: Q.1. What causes rancidity? Name an antioxidant which prevents rancidity. Q.2. Balance the equation: S + HNO3 → H2SO4 + NO2 + H2O Q.3. In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode. Q.4. Why respiration and decomposition processes are considered to be an exothermic process? Q.5. Identify the substance oxidised and reduced in the chemical reaction: 2Mg + O2 → 2MgO Q.6. Why potato chips manufacturers fill the packet of chips with nitrogen gas? Q.7. Identify the type of reaction: Fe + CuSO4(aq) → FeSO4(aq) + Cu(s) Q.8. Balance the given chemical equation: Al(s) + CuCl2(aq) → AlCl3(aq) + Cu(s) Q.9. What happens chemically when quicklime is added to water? Q.10. On adding dilute HCl to CuO powder, the solution formed is blue-green. Predict the new compound formed which imparts a blue-green colour to the solution. Q.11. What happens when: (a) copper powder is heated in a china dish? (b) hydrogen gas is passed over hot copper (II) oxide? Q.12. A student has found black coating on his silver coins and green coating on his copper coins. Which chemical phenomenon is responsible for this? Write chemical name of these coatings. Q.13. Give differences between: Oxidation and Reduction. Q.14. How combination reaction differs from decomposition reaction? Q.15. Differentiate between displacement and double displacement reactions. Q.16. What is observed when: (i) Potassium iodide is added to aqueous lead nitrate? (ii) Identify the type of reaction. (iii) Give a balanced chemical equation for this. Q.17. Define oxidising and reducing agents by giving suitable example. Q.18. What is meant by rancidity? How it can be prevented? Q.19. Give reasons why: (a) Magnesium ribbon should be cleaned before burning in air (b) Iron should be protected from moist air. Q.20. What happens when: (i) Lead nitrate is heated (ii) crystals of FeSO4 are heated? Q.21. Give two examples each of: (i) Double displacement reactions (ii) Redox reactions (iii) Combination reactions. Q.22. Differentiate between exothermic and endothermic reactions by giving one example of each. Q.23. Give reason why: (a) Na acts as an reducing agent while chlorine acts as an oxidising agent in the reaction: 2Na(s) + Cl2 (g) → 2NaCl(s) (b) White coloured AgCl turns grey when kept in sunlight. (c) Colour of the solution changes when iron nail is dipped in copper sulphate solution. (d) Lime water turns milky when CO2 is passed through it?

24 CHAPTER 02 ACIDS, BASES AND SALTS CHAPTER AT A GLANCE 1. Acids: Acids are the substances which have sour taste. They give H+ ions in aqueous solution and change the colour of blue litmus to red. They change the colour of natural indicators like turmeric, red onion peels, beetroot extract and coloured flower extract. Phenolphthalein (synthetic indicator) remains colourless in the presence of an acid. Methyl orange (synthetic indicator) becomes pink in the presence of an acid. 2. Qualitative analysis: It includes various chemical tests which are used to determine the chemical nature of salts. 3. Indicators: The substances which show different behaviour in acidic and basic medium and thus help in determining the chemical nature of the substance. Indicators show the presence of a chemical substance or ion by its colour. They are categorised into two categories. (i) Indicators showing different colours in acidic and basic medium. e.g., S.No. Name of the Colour in the Colour in the basic indicator medium (solution) medium (solution) 1.    Phenolphthalein Colourless Pink 2.    Methyl orange Pink Yellow (ii) Indicators giving different odours in acidic and basic medium. They are also called olfactory indicators. e.g., Onion, vanilla and clove oil have different odour in acidic and basic medium. 4. Natural indicators: Litmus is a natural indicator. Actually, litmus solution is a purple dye which is extracted from lichen. In acidic solution, it shows red colour whereas in basic solution, it shows blue colour. The other such natural indicator is the turmeric which is brown in basic medium and yellow in acidic medium. Some other such natural indicators are: (a) Red cabbage leaves. (b) Coloured petals of flowers such as, Hydrangea, Petunia and Geranium. Universal indicator: It is a mixture of a number of indicators which shows different colours at different pH values. 5. Reaction of acids with metals: ACID + METAL → SALT + HYDROGEN GAS More active metals like zinc, magnesium, calcium, etc., displaces hydrogen from the acids which can be seen as hydrogen gas. The metal combines with the remaining part of the acid and forms a compound called a salt. Active metal (s) + Dilute acid (aq) → Metal salt (aq) + Hydrogen (g) e.g., Mg (s) + H2SO4(aq) → MgSO4(aq) + H2(g ) Magnesium Sulphuric acid (dil.) Magnesium sulphate ( Salt ) Hydrogen 6. Reaction of metal carbonates and metal bicarbonates with acids: All metal carbonates and hydrogen carbonates react with acids to give a corresponding salt, carbon dioxide and water. e.g., Na2CO3(s) + 2HCl (aq) → 2NaCl (aq) + H2O (l) + CO2(g) Sodium carbonate Hydrochloric acid (dil.) Sodium chloride (Salt) Water Carbon dioxide

ACIDS, BASES AND SALTS 25 NaHCO3(s) + HCl (aq) → NaCl (aq) + H2O (l) + CO2 (g)   Sodium hydrogen carbonate Hydrochloric acid (dil.) Sodium carbonate Water Carbon dioxide When CO2 gas (which is evolved during the reaction) is passed through lime water. The following reaction takes place: Ca(OH)2 + CO2(g) → CaCO3(s) + H2O(l) Lime water White precipitate (Calcium hydroxide) (Calcium carbonate) When CO2 is passing in excess, the following reaction takes place: CaCO3 (s) + H2O(l) + CO2(g) → Ca(HCO3 )2 (aq) Calcium carbonate Calcium hydrogen carbonate (White precipitate) (Soluble in water) In the similar way, different forms of calcium carbonate (CaCO3) such as: limestone, chalk and marble reacts with acids. CaCO3(s) + H2SO4(aq) → CaSO4(aq) + H2O (l) + CO2(g ) Calcium carbonate Sulphuric acid (dil.) Calcium sulphate Water Carbon dioxide (also called as limestone/chalk/marble) 7. Reaction of Acids and Bases with each other: BASE + ACID → SALT + WATER The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. Generally, heat is given out during this reaction and the acidic properties of the acid disappear. e.g., HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l) Hydrochloric acid (Acid) Sodium hydroxide (Base) Sodium chloride (Salt) Water 8. Reaction of Metallic oxides with acids: Metallic oxides are said to be basic oxides. Similar to the reaction of a base with an acid, metallic oxides react with acids to give salts and water. METAL OXIDE + ACID → SALT + WATER e.g., When copper oxide is added to dilute hydrochloric acid (with constant stirring), it results in the formation of copper (II) chloride which can be known by change of blue-green colour of the solution. CuO(s) + 2HCl (aq) → CuCl2(aq) + H2O (l) Copper Water (II) oxide Hydrochloric acid Copper (II) chloride (Blue-green colour) 9. Reaction of a Non-metallic oxide with base: NON-METALLIC OXIDE + BASE → SALT + WATER Non-metallic oxides react with base to produce a salt and water. This reaction is similar to the reaction between a base and an acid. This proves that, ‘Non-metallic oxides are acidic in nature’. e.g., Ca(OH)2 + CO2 → CaCO3 + H2O 10. Remember these facts: (a) The electric current is carried through the solution by ions. (b) Solutions of glucose and alcohol do not conduct electricity indicating that they do not form ions. (c) Acids produce hydrogen ions, H+(aq), which are responsible for the acidic properties of all acids. (d) When any concentrated acid is diluted, then the concentration of H+(aq) ions per unit volume decreases and thus, the strength of an acid decreases. (e) Ethyl alcohol (C2H5OH) contains hydroxyl group (–OH). But since it does not ionise in the aqueous solution to give OH–(hydroxide)ions, hence, ethyl alcohol cannot be termed as a base. (f) The aqueous solution of bases conduct electricity.

26 ACIDS, BASES AND SALTS 11. Bases: Bases are bitter and change the colour of the red litmus to blue. A base is a substance which gives hydroxide (OH+)ions in aqueous solutions. Remember this fact:  All metal hydroxides are bases and metal oxides show basic behaviour. This suggests that, ‘BASES ARE OXIDES AND HYDROXIDES OF METALS’. To this definition, ammonium hydroxide (NH4OH) is an exception.  The bases which are soluble in water and give hydroxide ions in the aqueous solution are called alkalies. These two terms (bases and alkalies) are generally used to define a substance which contain hydroxide ions in it. But care should be taken to distinguish between the two. Base is a substance which contains hydroxide ions in it while Alkali is a water soluble base. Therefore, it can be said: “All alkalies are bases but all bases are not alkalies”. 12. Reaction of Bases with Metals: BASE + Metal → SALT + HYDROGEN GAS (Strong) (More active) 2NaOH (aq) + Zn (s) → Na2ZnO2 (aq) + H2(g) Sodium hydroxide Zinc (Metal) Sodium zincate (Salt) Hydrogen (Base) 2NaOH (aq) + 2Al (s) + 2H2O(l) → 2NaAlO2(aq) + 3H2(g) Sodium hydroxide Aluminium Water Sodium aluminate Hydrogen Note: The reaction of bases with metals are possible only for few metals. 13. Behaviour of an Acid in a water (aqueous) solution : An acid produces hydrogen ions only in the presence of water. The separation of H+ (hydrogen) ion from hydrochloric acid (HCl) molecules cannot occur in the absence of water. HCl(g) Dry medium→ H+ + Cl– (This reaction cannot occur). HCl(g) + Organic solvent like benzene, alcohol, etc. Dry medium→ H++Cl– (This reaction also cannot occur). HCl + H2O Aqueousmedium→ H3O+ + Cl– (This reaction takes place easily). This suggests that, “ Water molecules have the ability to pull the H+ (hydrogen) and Cl–(chloride) ions”. Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus, hydrogen ions must be shown as H+(aq) or hydronium ion (H3O+). H+ + H2O → H3O+ It can be said that, “Acids give H3O+ or H+ ions in water”. And properties of all acids are due to the presence of these ions which they furnish in aqueous (water) solution. 14. Behaviour of a base in a water (aqueous) solution: NaOH(s) AqueoHus2Omedium→ Na+(aq) +OH–(aq) KOH(s) → K+(aq) + OH– (aq) Mg(OH)2(s) H2O→ Mg2+(aq) + 2OH–(aq) Bases generate hydroxide ions (OH–) in water. We have already learnt that, ‘Bases which are soluble in water are called alkalies’. Remember these facts about bases: (a) All bases do not dissolve in water. (b) An alkali is a base that dissolves in water. (c) Bases are soapy to touch, bitter and corrosive. (d) Bases turn red litmus blue and phenolphthalein solution pink. (e) The aqueous solution of bases conduct electricity.

ACIDS, BASES AND SALTS 27 (f) Bases react with acids to form salt and water. (g) Strong bases react with metals to form salt and hydrogen gas. (h) Bases react with non-metallic oxides (CO2, SO2) to form salt and water. Neutralisation reaction of a base: Acid + Base → Salt + Water HX + MOH → MX + HOH H+(aq) + OH–(aq) → H2O(l) 15. Dissolving an acid or a base in water: The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid. The heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted. 16. Warning sign displayed on containers containing concentrated acids and bases. This sign shows that ‘the given substance is corrosive in nature’. 17. Measuring strength of an acidic or basic solutions : It can be measured by making use of a universal indicator. Which is a mixture of several indicators. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution. 18. pH scale: A scale for measuring hydrogen ion concentration in a solution is called pH scale. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale, we can measure pH from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration in the solution, that is, increase in the strength of alkali. Generally, paper impregnated with the universal indicator is used for measuring pH. 19. Strength of an Acid: The strength of acids and bases depends on the number of H+ ions and OH– ions produced, respectively. If we take hydrochloric acid and acetic acid of the same concentration, say one molar, then these produce different amounts of hydrogen ions. Acids that give rise to more H+ ions are said to be strong acids and acids that give less H+ ions are said to be weak acids.

28 ACIDS, BASES AND SALTS Strong acids: The acids which when dissolved in water dissociate completely into ions. e.g., HCl, HNO3, H2SO4. Weak acids: The acids which when dissolved in water dissociate partially in the solution and produce very little H+ ions. e.g., CH3COOH (acetic acid); HCOOH (formic acid); H2CO3 (carbonic acid) and H3PO4 (phosphoric acid). Noxoytgee: nInatCoHm3CgeOtsOioHniasnedd HCOOH (commonly known as organic acids), only the hydrogen atom attached to and the hydrogen atoms attached to carbon atom remains unaffected. 20. Importance of pH in Everyday life: (a) In living organisms: They work in the pH range of 7·0 to 7·8. (b) In aquatic organisms: The survival of aquatic organisms become difficult when acid rain falls into the rivers which lowers the pH to 5·6. (c) In plants: Plants grow in such soil which is neither alkaline nor highly acidic. (d) In our digestive system: In our stomach, HCl is secreted which helps in breakdown of food particles without harming the stomach. But too much of acid causes pain and irritation. This leads to acidity. It can be cured by using antacids, such as magnesium hydroxide (milk of magnesia) which is a mild base and thus neutralises the excess of acid. (e) In causing tooth decay: When pH of mouth is lower than 5·5, the tooth decay starts. At lower pH, tooth enamel (hardest substance in our body) which is made of calcium phosphate gets eroded. Bacteria (present in the mouth) produce acids by decaying sugar and food particles (which remain in the mouth after eating meals). Remedy: Tooth decay can be prevented by using two methods: (i) Cleaning mouth (after every meal) with saline water. (ii) Using toothpaste (which are mostly basic) which can neutralise the excess acid. (f) Self-defence of animals and plants through chemical warfare: (i) Bee-sting: It leaves an acid (methanoic acid) causing pain and irritation. Remedy: Using milk base like baking soda on the stung area gives relief. (ii) Stinging hair of neetle plants: These hair inject methanoic acid which causes burning pain. Remedy: By rubbing the area with the leaf of the dock plant (which grows beside the neetle plant in the wild). 21. Some naturally occurring acids: The word ‘ACID’ is derived from the Latin word ‘ACIDUS’ which means ‘SOUR’. The sour taste of many food items is due to the presence of some acids in them. e.g., S.No. Natural source of the acid Name of the acid 1. Vinegar Acetic acid 2. Orange, lemon Citric acid 3. Tamarind, grapes Tartaric acid 4. Tomato Oxalic acid 5. Sour milk (curd) Lactic acid 6. Ant sting, neetle sting Methanoic acid 7. Olive oil Oleic acid 8. Gastric juice Hydrochloric acid 9. Apples (green) Malic acid 10. Fats Stearic acid 11. Rancid butter Butyric acid 12. Urine Uric acid 13. Fresh fruit and vegetables Ascorbic acid (vitamin C)

ACIDS, BASES AND SALTS 29 14. Bark, wood (of tree) and tea Tannic acid 15. Green leafy vegetables Folic acid 22. Salts: Salts are the ionic compounds formed by the combination of positive charge (called cation) and the negative charge (called anion). Salts are electrically neutral. Generally, the positive ion is a metal ion excepting ammonium ion (NH4+) which is a non-metallic ion. Salts are of three categories: (i) Salts of a strong acid and a strong base: They are neutral with a pH value of 7. These salts produce a strong base and strong acid when dissolved in water. e.g., NaCl [NaOH + HCl] Na2SO4 [NaOH + H2SO4] KNO3 [KOH + HNO3] (ii) Salts of a strong acid and a weak base: They are acidic with a pH value less than 7. These salts produce a strong acid and a weak base when dissolved in water. e.g., NH4Cl [NH4OH + HCl] CuSO4 [Cu(OH)2 + H2SO4] AlCl3 [Al(OH)3 + HCl] (iii) Salts of a strong base and a weak acid: They are basic in nature, with pH value more than 7. These salts produce a strong base and a weak acid when dissolved in water. e.g., Na2CO3 [NaOH + H2CO3] CH3COONa [NaOH + CH3COOH] 23. Common salt: It acts as a raw material for various materials of daily use, such as : NaOH (Caustic soda) NaHCO3 (Baking soda) Na2CO3 (Washing soda) CaOCl2 (Bleaching powder) 24. Preparing NaOH from NaCl: NaOH is prepared by passing electricity through an aqueous solution of NaCl (called brine). The process is called CHLOR-ALKALI process (chlor for chlorine and alkali for sodium hydroxide–an alkali). During the process, NaCl decomposes to form: (i) Chlorine gas at the anode. (ii) Hydrogen gas at the cathode. (iii) NaOH near the cathode. 25. Preparing Bleaching powder from NaCl : The chlorine gas produced during the chlor-alkali process is used for manufacturing bleaching powder by the action of chlorine gas on dry slaked lime. Ca(OH)2 + Cl2 → CaOCl2 + H2O Slaked lime Chlorine (Dry) Calcium oxychloride (Bleaching powder) Uses of CaOCl2: (i) For bleaching cotton, linen, wood pulp and washed clothes in laundry. (ii) As an oxidising agent in chemical industries. (iii) For disinfecting water (to make it free from germs). 26. Preparing Baking soda from NaCl: Baking soda (NaHCO3) is prepared by Solvay process using sodium chloride as one of the raw materials. NaCl + H2O + CO2 + NH3 → NH4Cl NaHCO3 + Ammonium chloride Sodium hydrogen carbonate (Baking soda)

30 ACIDS, BASES AND SALTS Uses of NaHCO3: (i) In soda-acid fire extinguishers. (ii) In antacids, as it neutralises excess acid in the stomach. It shows NaHCO3 is alkaline in nature. H+ NaHCO3 + → CO2 + H2O + Sodium salt of an acid (From any acid) (iii) For making baking powder (which is a mixture of NaHCO3 and a mild edible acid like tartaric acid).   27. Preparing washing soda from NaCl: When NaHCO3 is decomposed, it forms Na2CO3 and when Na2CO3 is recrystallised, it gives washing soda (a basic salt). 2NaHCO3 Heat → Na2CO3 + H2O + CO2 Sodium hydrogen Sodium carbonate carbonate Na2CO3 + 10H2O → NaCO3 .10H2O Washing soda Uses of washing soda: (i) It is used in glass, soap and paper industry. (ii) It is used for making borax. (iii) It is used for removing permanent hardness. (iv) It is used as a cleaning agent for domestic purposes. 28. Water of Crystallisation: It means the number of water molecules which are required for the formation of crystals of any salt. Thus, water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. e.g., CuSO4.5H2O (It means that 5 water molecules are present in one formula unit of CuSO4) Na2CO3.10H2O (Washing soda has 10 water molecules in one formula unit of Na2CO3) CaSO4.2H2O (Gypsum has 2 water molecules in one formula unit of CaSO4) 29. Plaster of Paris: When gypsum is heated at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO4. 12 H2O) which is called Plaster of Paris. Property: It is a white powder which on mixing with water, changes into gypsum (once again giving a hard solid mass). CaSO4 , 12H2O + 121 H2O → CaSO4 , 2H2O Plaster of Paris Gypsum Uses of Plaster of Paris: (i) It is used as plaster for supporting fractured bones in the right position. (ii) For making toys and materials for decoration. (iii) For making smooth surfaces. Name of Indicator Colour in The Medium Acidic Neutral Basic Methyl orange Orange Pink Yellow Phenolphthalein Colourless Colourless Light pink/Magenta Litmus Purple Red Blue Red cabbage leave extract Red Red Green Flowers of hydrangea plant Blue Blue Pink Beetroot Dark pink Pink Pale yellow China rose Pink Dark pink Green Turmeric Yellow Yellow Brownish-red

ACIDS, BASES AND SALTS 31 OLFACTORY INDICATOR Onion Vanilla Smell in acidic and neutral medium and no smell in basic medium. Clove oil NOTES

32 ACIDS, BASES AND SALTS ASSIGNMENT (In Class Discussion) Q.1. Which of the following is a synthetic indicator? Options: (a) Litmus (b) China rose solution (c) Turmeric solution (d) Phenolphthalein Q.2. What will be the colour of phenolphthalein in sodium hydroxide solution? Options: (a) Yellow (b) Pink (c) Orange (d) Colourless Q.3. Which of the following statements is true for acids? Options: (a) Bitter and change red litmus to blue (b) Sour and change red litmus to blue (c) Sour and change blue litmus to red (d) Bitter and change blue litmus to red Q.4. Which of the following phenomena occur when a small amount of an acid is added to water? (i) Ionisation (ii) Neutralisation (iii) Dilution (iv) Salt formation Options: (a) (i) and (ii) (b) (i) and (iii) (c) (ii) and (iii) (d) (ii) and (iv) Q.5. Which of the following are present in a dilute aqueous solution of hydrochloric acid? Options: (a) H3O+ + Cl–(aq) (b) H3O+ / OH– (c) Cl– + OH– (d) unionised HCl Q.6. In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus was set up: Which among the following statement(s) is(are) correct? (i) Bulb will not glow because electrolyte is not acidic. (ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction. (iii) Bulb will not glow because the circuit is incomplete. (iv) Bulb will not glow because it depends upon the type of electrolytic solution. Options: (a) (i) and (iii) (b) (ii) and (iv) (c) (ii) only (d) (iv) only Q.7. Which of the following is (are) true when HCl gas is passed through water? (i) It does not ionise in the solution as it is a covalent compound. (ii) It ionises in the solution (iii) It gives both hydrogen and hydroxyl ion in the solution

ACIDS, BASES AND SALTS 33 (iv) It forms hydronium ions in the solution due to the combination of hydrogen ions with water molecules. Options: (a) (i) only (b) (iii) only (c) (ii) and (iv) (d) (iii) and (iv) Q.8. Which one of the following can be used as an acid-base indicator by visually impaired people? Options: (a) Litmus (b) Turmeric (c) Vanilla essence (d) Petunia leaves Q.9. What would be the colour of red litmus in a solution of sodium carbonate? Q.10. State reason for the following statement: Stain of curry on a white cloth becomes reddish brown when soap is scrubbed on it and turns yellow again when the cloth is washed with plenty of water. Q.11. What will be the action of the following substances on litmus paper? Lemon juice, Carbonated soft drink, Curd, Soap solution Q.12. Name the acid present in the following: (i) Tomato (ii) Vinegar (iii) Tamarind (iv) Curd Q.13. Explain the action of dilute hydrochloric acid on the following with the help of balanced chemical equations: (a) Magnesium ribbon (ii) Sodium hydroxide (iii) Crushed egg shells Q.14. Equal lengths of magnesium ribbon are taken in two test tubes ‘A’ and ‘B’. Dilute H2SO4 is added to test tube ‘A’ and dilute H2CO3 in the test tube ‘B’ in equal amounts: (i) Identify the test tube showing vigorous reaction. (ii) Give reason to support your answer. (iii) Name the gas liberated in both the tubes. How will you prove its liberation? (iv) Write chemical equations for both reactions. Q.15. A student dropped a few pieces of marble in dilute hydrochloric acid contained in a test-tube. The evolved gas was then passed through lime water. What change would be observed in lime water? What will happen if excess of gas is passed through lime water? Explain the observations with the help of balanced chemical equations for all the changes. Q.16. To protect our teeth from decay, we are advised to brush our teeth regularly. The nature of the toothpaste commonly used is: Options: (a) acidic (b) neutral (c) basic (d) corrosive Q.17. Which of the following statements are correct about an aqueous solution of an acid and of a base? (i) Higher the pH, stronger the acid (ii) Higher the pH, weaker the acid (iii) Lower the pH, stronger the base (iv) Lower the pH, weaker the base Options: (a) (i) and (iii) (b) (ii) and (iii) (c) (i) and (iv) (d) (ii) and (iv) Q.18. The pH of the gastric juices released during digestion is: Options: (a) less than 7 (b) more than 7 (c) equal to 7 (d) equal to 0 Q.19. Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the colour obtained? (You may use the colour guide given in Figure).

34 ACIDS, BASES AND SALTS Options: (a) Red (b) Yellow (c) Yellowish green (d) Blue Q.20. Common salt besides being used in the kitchen, can also be used as the raw material for making which of the following? (i) Washing soda (ii) Bleaching powder (iii) Baking soda (iv) Slaked lime Options: (a) (i) and (ii) (b) (i), (ii) and (iv) (c) (i) and (iii) (d) (i), (iii) and (iv) Q.21. One of the constituents of baking powder is sodium hydrogen carbonate. The other constituent is: Options: (a) Hydrochloric acid (b) Tartaric acid (c) Acetic acid (d) Sulphuric acid Q.22. Match the chemical substances given in Column (A) with their appropriate application given in Column (B) Column (A) Column (B) (A) Bleaching powder (i) Preparation of glass (B) Baking soda (ii) Production of H2 and Cl2 (C) Washing soda (iii) Bleaching (D) Sodium chloride (iv) Antacid Options: (a) A — (ii), B — (i), C — (iv), D — (iii) (b) A — (iii), B — (ii), C — (iv), D — (i) (c) A — (iii), B — (iv), C — (i), D — (ii) (d) A — (ii), B — (iv), C — (i), D — (iii) Q.23. Identify the correct representation of reaction occurring during the chlor-alkali process. Options: (a) 2 NaCl (l) + 2 H2O (l) → 2 NaOH (l) + Cl2 (g) + H2 (g) (b) 2 NaCl (aq) + 2 H2O (aq) → 2 NaOH (aq) + Cl2 (g) + H2 (g) (c) 2 NaCl (aq) + 2 H2O (l) → 2 NaOH (aq) + Cl2 (aq) + H2 (aq) (d) 2 NaCl (aq) + 2 H2O (l) → 2 NaOH (aq) + Cl2 (g) + H2 (g) Q.24. State the chemical properties on which the following uses of baking soda are based: (i) As an antacid. (ii) As a soda acid fire extinguisher. (iii) To make bread and cake soft and spongy. Q.25. (i) Give the chemical formula of bleaching powder. (ii) Why does cake or bread swell on adding baking powder? Write chemical equations. Q.26. “Sodium hydrogen carbonate is a basic salt.” Justify the statement. How is it converted into washing soda? Explain.

ACIDS, BASES AND SALTS 35 Q.27. (a) The pH of soil A is 7.5 while that of soil B is 4.5. Which of the two soils A or B should be treated with powdered chalk to adjust its pH and why? (b) Name the chemical which is injected into the skin of a person: (i) During an ant’s sting (ii) During the nettle leaf sting. How can the effect of these stings be neutralised? (c) Explain how the pH change in the river water can endanger the lives of aquatic animals like fish? Q.28. You have four solutions A, B, C and D. The pH of solution A is 6, B is 9, C is 12 and D is 7. (a) Identify the most acidic and most basic solutions. (b) Arrange the above four solutions in the increasing order of H+ ion concentration. (c) State the change in colour of pH paper on dipping in solution C and D. Q.29. (a) Explain how antacids give relief from acidity. Write the name of one such antacid. (b) Fresh milk has a pH of 6. How does the pH change as it turns into curd? Explain your answer. (c) A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a longer time to set as curd? Q.30. The soil in a field is highly acidic. List any two materials which can be added to this soil to reduce its acidity. Give the reason for your choice. NOTES

36 ACIDS, BASES AND SALTS ASSERTIONS AND REASONING TYPE QUESTIONS (Self-Practice) I. Directions: In the following questions, two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below: (a) Both (A) and (R) are true and (R) is correct explanation of the assertion. (b) Both (A) and (R) are true but (R) is not the correct explanation of the assertion. (c) (A) is true but (R) is false. (d) (A) is false but (R) is true. Q.1. Assertion (A): When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed. Reason (R): When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen. Q.2. Assertion (A): When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives the corresponding salt, carbon dioxide gas and water. Reason (R): Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively. Q.3. Assertion (A): Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. Reason (R): Acidic nature of a substance is due to the formation of H+(aq) ions in solution. Formation of OH– (aq) ions in solution is responsible for the basic nature of a substance. Q.4. Assertion (A): Mixing concentrated acids or bases with water is a highly exothermic process. Reason (R): Acids and bases neutralise each other to form corresponding salts and water. Q.5. Assertion (A): The strength of an acid or an alkali can be tested by using a scale called the pH scale (0-14) which gives the measure of hydrogen ion concentration in a solution. Reason (R): A neutral solution has a pH of exactly 7, while an acidic solution has a pH less than 7 and a basic solution a pH more than 7. II. Directions: In the following questions, two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below: (a) Both (A) and (R) are true and (R) is correct explanation of the assertion. (b) Both (A) and (R) are true but (R) is not the correct explanation of the assertion. (c) (A) is true but (R) is false. (d) (A) is false but (R) is true. Q.1. Assertion (A): Copper sulphate crystals turns white on heating. Reason (R): Copper sulphate contains water of crystallisation. Q.2. Assertion (A): Plaster of Paris should be stored in moisture proof container. Reason (R): Plaster of Paris changes to gypsum in the presence of moisture. Q.3. Assertion (A): Ammonia is a base. Reason (R): It does not contain OH– ions. It can donate a lone pair of electron (according to Lewis concept).

ACIDS, BASES AND SALTS 37 Q.4. Assertion (A): Calcium sulphate hemihydrate, CaSO4. 1 H2O is called plaster of Paris. 2 Reason (R): Plaster of Paris is used for producing moulds for pottery and ceramics and casts of statues. Q.5. Assertion (A): Bleaching powder reacts with dilute acids to evolve chlorine. CaOCl2 + H2SO4 → CaSO4 + H2O + Cl2↑ Reason (R): The chlorine liberated by the action of dilute acids on bleaching powder is called available chlorine. Q.6. Assertion (A): The strength of an acid or an alkali can be tested by using a scale called the pH scale (0 – 14) which gives the measure of hydrogen ion concentration in a solution. Reason (R): A neutral solution has a pH of exactly 7, while an acidic solution has a pH less than 7 and a basic solution a pH more than 7. NOTES

38 ACIDS, BASES AND SALTS QUESTION BANK Short Answer Type Questions (3 Marks) Q.1. Tooth enamel is the hardest substance in our body. Name the compound of which it is made-up of. At what pH of the mouth it gets corroded? State the role of bacteria present in the mouth. Suggests a method to prevent tooth decay. Q.2. (i) What is the pH range for a base? (ii) How the strength of basic solution can be increased? (iii) Give an example of strong and weak base. Q.3. Acids do not show acidic behaviour in the absence of water but do so in water. Justify this statement with the help of a chemical equation. Q.4. What happens when: (a) CO2 is passed through lime water in limit? (b) CO2 is passed through lime water in excess? Q.5. Crystals of copper sulphate are heated in a test tube for sometime. (a) What is the colour of copper sulphate crystals: (i) b efore heating, and (ii) after heating? (b) What is the source of liquid droplets seen on the inner upper side of the test tube during the heating process? Q.6. How is bleaching powder prepared? Why does bleaching powder: (i) smell strongly of chlorine? (ii) not dissolve completely in water? Q.7. Write the chemical name of Na2CO3.10H2O and Na2CO3. Write the significance of 10H2O. Mention the term used for water molecules attached with a salt. Q.8. 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed, a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid. Q.9. The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses. Q.10. A compound ‘X’ is a constituent of baking powder. It is used as an antacid. When ‘X’ is heated it gives out a ‘Y’ which, when passed through lime water turns it milky and salt ‘Z’ is formed which is the main constituent of washing power. Identify X, Y and Z. Write balanced chemical equations for the reactions involved. Q.11. A solution of a substance ‘X’ is used for testing carbon dioxide. Identify ‘X’. What will be its reaction with carbon dioxide? Write a balanced equation for the reaction. Q.12. (i) State the purpose of developing pH scale. (ii) Mention the pH range for acids. (iii) What is the pH of acid rain and how it affects the aquatic life? Q.13. (a) If we take hydrochloric acid and acetic acid of same concentration, which one will produce less H+ ion concentration? Out of two which one is weak acid? (b) If someone is suffering from acidity, which of the following would you suggest in order to cure it: Vinegar, orange juice, baking soda solution? Give reason for your answer.

ACIDS, BASES AND SALTS 39 Q.14. Complete and balance the following equations: (a) Ca(OH)2 + Cl2 → (b) CaSO4. ½ H2O + H2O → (c) Na2CO3.10H2O → Q.15. (a) Give the chemical names of acids present in : (i) ants (ii) lemon (iii) milk (iv) tomato (b) Write the chemical names of two salts belonging so sodium family. Q.16. Explain why: (a) colour of copper sulphate crystals changes on heating? (b) baking soda acts as an antacid? (c) an acid should be added to water while diluting? Q.17. Write the balanced equation involved, when: (a) chlorine is passed over dry slaked lime. (b) sodium bicarbonate reacts with dilute hydrochloric acid. (c) sodium bicarbonate is heated. Q.18. (a) Three acidic solutions A, B and C have pH = 0, 3 and 5 respectively. (i) Which solution has the highest concentration of H+ ions? (ii) Which solution has the lowest concentration of H+ ions? (b) Why does 1 M HCl solution have a high concentration of H+ ions than 1M CH3COOH solution? Q.19. (a) How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal? (b) In which form H+ (hydrogen) ions exist in water ? (c) How is the pH of a solution of an acid influenced when it is diluted? Q.20. (a) A metal compound X reacts with dilute hydrochloric acid to produce brisk effervescence. The gas evolved forms a white precipitate when passed through lime water. Write balanced chemical equations involved in the above mentioned chemical reactions. (b) Give the chemical name and formula of bleaching powder. Q.21. (a) What does pH scale measure? (b) Write its range. (c) State the significance of highest and lowest values of pH scale. Q.22. A compound which is prepared from gypsum has the property of hardening when mixed with a proper quantity of water. Identify the compound and write its chemical formula. Write its chemical equation for its preparation. Mention any one use of the compound. Q.23. A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound B is also a by-product of chlor-alkali process. Identify B. State the type of reaction that occurs when B is treated with an acidic oxide, say sulphur dioxide. Also write chemical equation for the reaction involved. Q.24. Write the chemical formula of washing soda. How can it be obtained from baking soda? List two industries in which washing soda is used for other purposes than washing clothes. Q.25. Identify the acid and the base from which sodium chloride is obtained. Which type of salt is it? When is it called rock salt? How is rock salt formed? Long Answer Type Questions (5 Marks) Q.1. (a) Write the chemical formula of hydrated copper sulphate and anhydrous copper sulphate. Giving an activity illustrate how these two are inter convertible.

40 ACIDS, BASES AND SALTS (b) Write chemical names and formulae of plaster of paris and gypsum. Q.2. Write balanced chemical equations for the following: (i) Bleaching powder is kept open in air. (ii) Blue crystals of copper sulphate are heated. (iii) Chlorine gas is passed through dry slaked lime. (iv) Carbon dioxide gas is passed through lime water. (v) NaOH solution is heated with zinc granules. Q.3. Equal lengths of magnesium ribbon are taken in two test tubes A and B. H2SO4 is added to test tube A and H2CO3 in test tube B in equal amounts. (a) Identify the test tube showing vigorous reaction. (b) Give reason to support your answer. (c) Name the gas liberated in both the test tubes. How will you prove its liberation? (d) Write chemical equations for both the reactions. (e) Out of the two acids taken above, which one will have lower pH value and lower H+ concentration respectively? Q.4. How do metal carbonates and metal hydrogen carbonates react with acids? Give their chemical equations. Name the gas evolved during the reaction. What will happen when this gas is passed through lime water? Q.5. The metal salt A is blue in colour. When salt A is heated strongly over a burner, then a substance B present in it is eliminated and a white powder C is left behind. When a few drops of a liquid D are added to powder C, it becomes blue again. (a) Identify A, B, C, D. (b) Write the chemical equations involved. (c) Give an example of the salt which also shows above property. Q.6. (a) Write the chemical name and chemical formula of washing soda. (b) How is it obtained from sodium chloride? Give equations of the reactions. (c) Why is it called a basic salt? Give its any one use. Q.7. (a) Identify the acid and the base whose combination forms the common salt that you use in your food. Write its formula and chemical name of this salt. Name the source from where it is obtained. (b) What is rock salt? Mention its colour and the reason due to which it has this colour. (c) What happens when electricity is passed through brine? Write the chemical equation for it. NOTES

ACIDS, BASES AND SALTS 41 APPLICATION BASED QUESTIONS (Self-Practice) Q.1. What is the nature of the salt if the pH of its solution is greater than 7? Name the acid and base would be used to prepare the following salts: (i) Potassium sulphate. (ii) Ammonium chloride. Q.2. A metal compound ‘A’ reacts with dilute sulphuric acid to produce a gas which extinguishes a burning candle. Identify the compound ‘A’ and the gas produced. Write a balanced chemical equation for the reaction if one of the compounds formed in the reaction is sodium sulphate. Q.3. You have two solutions A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which one of this is acidic and which one is basic? Q.4. Give suitable reasons for the following statements: An aqueous solution of sodium chloride is neutral but an aqueous solution of sodium is basic. Q.5. State the chemical property in each of the following cases, on which the following uses of baking soda are based: (a) Applied on an ant stung area. (b) As a constituent of baking powder. (c) In soda-acid fire extinguisher. Q.6. Differentiate between strong electrolytes and weak electrolytes. Mention two examples each. Q.7. When soap is scrubbed on a stain of curry on a white cloth, why does it become reddish brown, and turns yellow again when the cloth is washed with plenty of water? Q.8. Explain the action of dilute hydrochloric acid on the following with chemical equations: (a) Magnesium ribbon (b) Sodium hydroxide (c) Crushed egg shells Q.9. (a) A solution of a substance ‘X’ is used for testing carbon dioxide. Write the equation of the reaction ‘X’ with carbon dioxide. (b) How is ‘X’ obtained? Write chemical equation. NOTES

42 ACIDS, BASES AND SALTS ANALYZING, EVALUATING & CREATING TYPE QUESTIONS (Self-Practice) Q.1. Answer the following: (i) Why is Plaster of Paris written as CaSO4.½H2O ? How is it possible to have half a water molecule attached to CaSO4? (ii) Why is sodium hydrogen carbonate an essential ingredient in antacids? (iii) When electricity is passed through an aqueous solution of sodium chloride, three products are obtained. Why is the process called chlor-alkali? Q.2. Give reason why: (i) it is recommended that the acid should be added to water and not water to the acid? (ii) compounds like alcohol and glucose do not show acidic behaviour although they contain hydrogen in them? Q.3. Tap water conducts electricity whereas distilled water does not. Why? Q.4. Dry hydrogen chloride gas does not turn blue litmus red whereas hydrochloric acid does. Give one reason. NOTES

ACIDS, BASES AND SALTS 43 CHAPTER TEST (Self-Practice) Time: 1½ Hour Maximum Marks: 45 PARAnswer the following Questions: Q.1. Which one of the following has a higher concentration of H+ions? 1M HCl or 1M CH3COOH? Q.2. Why CH3COOH is considered a weak acid whereas HCl is considered as a strong acid? Q.3. Which gas is evolved when dil.HCl reacts with metal bicarbonate? How is it recognised? Q.4. What effect does an increase in concentration of H+ (aq) ions in a solution have on the pH of a solution ? Q.5. Name the gas evolved when dil. HCl reacts with an active metal. How is it recognised? Q.6. Why does tooth decay start when the pH of mouth is lower than 5.5? Q.7. How is the pH of a solution of an acid influenced when it is diluted? Q.8. Fresh milk has a pH of 6. When it changes into curd (yogurt), will its pH value increases or decreases? Why ? Q.9. Arrange the pH values of 0, 2, 4 and 6 in the increasing value of H+ (aq) ions conc.? Q.10. Why the flow of acid rain water into a river make the survival of aquatic life in the river difficult? Q.11. Crystals of CuSO4 are heated in a test tube for sometime. (a) What is the colour of CuSO4 crystals: (i) before heating, and (ii) after heating? (b) What is the source of liquid droplets seen on the inner upper side of the test tube during the heating process? Q.12. (i) Name the products formed when NaHCO3 is heated. (ii) Write the chemical equation for the reaction involved in the above. Q.13. Write the name and chemical formula of the calcium compound used for disinfecting drinking water. How is this compound manufactured? Q.14. Show the decomposition of gypsum to P.O.P. and back interconversion of P.O.P. to gypsum. Q.15. What happens when: (a) CO2 is passed through lime water in a limited quantity? (b) CO2 is passed through lime water in excess? Q.16. Give a reason to explain why: (a) bleaching powder smell strongly of chlorine? (b) bleaching powder does not dissolve completely in water? Q.17. What is the chemical formula for washing soda? How can it be obtained from baking soda? Describe two applications of washing soda. Q.18. What is the difference between litmus solution indicator and universal indicator solution? How synthetic indicators differ from natural indicators? Q.19. What happens when: (a) zinc reacts with NaOH (b) zinc reacts with HCl? Q.20. Give two characteristics each of acids and bases. Q.21. Explain chlor-alkali process. Why is it called so? Q.22. Give reason why: (a) water should not be added to concentrated acid? (b) antacids are required when there is pain or irritation in the stomach? (c) baking soda should be rubbed on bee-stung area? Q.23. What is meant by water of crystallisation? Name four such salts which contain water of crystallisation in them. Why P.O.P. is written as CaSO4 .½ H2O? Q.24. Why NaHCO3 is used in soda-acid fire extinguisher as well as for making baking powder.

44 ACIDS, BASES AND SALTS Q.25. Give reason why: (a) tartaric acid is added while making baking powder ? (b) bleaching powder is used for disinfecting drinking water? (c) strong acid and concentrated acid does not mean the same thing? NOTES

45 CHAPTER 03 METALS AND NON-METALS CHAPTER AT A GLANCE 1. Metals: All those elements which form positive ions by losing electrons are called metals. They have 1 to 3 electrons in their valence shell. 2. Physical properties of metals: (a) Metallic lustre: Metals have shinning surface. Aluminium and magnesium appear white, gold is yellow and copper is reddish-brown in colour. (b) Hardness: This varies from metal to metal. Metals are generally hard. Sodium and potassium are so soft that they can be easily cut by a knife. (c) Malleability: Some metals can be beaten into thin sheets, e.g., gold and silver. (d) Ductility: The ability of metals to be drawn into thin wires is called ductility. Gold is most ductile metal. (e) Conductivity of heat and electricity: Metals are good conductors of heat and electricity. They have high melting points. Silver and copper are best conductors of heat and electricity. Mercury is bad conductor of them. (f) Sonority: Metals are sonorous, i.e., they produce a sound on striking a hard surface. (g) Metals are solid at room temperature except mercury and have high densities except Na and K. 3. Chemical properties of metals: (a) Electropositive: Metals are electropositive in nature as they form positive ions by loss of electrons. (b) Reaction with oxygen: METAL + OXYGEN → METAL OXIDE Metal oxides are basic in nature. Some metal oxides, such as aluminium oxide and zinc oxide show both acidic and basic behaviour. These are known as amphoteric oxides. When copper is heated in air, it combines with oxygen to form copper(II) oxide, a black oxide. 2Cu + O2 → 2CuO (Copper) (Copper (II) oxide) (Black) Aluminium forms aluminium oxide. 4Al + 3O2 → 2Al2O3 (Aluminium oxide) (Aluminium) Aluminium oxide reacts in the following manner with acids and bases: Al2O3 + 6HCl → 2AlCl3 + 3H2O Al2O3 + 2NaOH → 2NaAlO2 + H2O (Sodium aluminate) Sodium and potassium are kept immersed in kerosene oil because of their vigorous reaction with oxygen. (c) Reactions with water: Metals react with water and produce a metal oxide and hydrogen gas. Metal oxides that are soluble in water dissolve in it to further form metal hydroxide. But all metals do not react with water. Metal + Water → Metal oxide + Hydrogen Metal oxide + Water → Metal hydroxide

46 METALS AND NON-METALS Metals like potassium and sodium react violently with cold water. In case of sodium and potassium, the reaction is so violent and exothermic that the evolved hydrogen immediately catches fire. 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) + Heat energy 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) + Heat energy The reaction of calcium with water is less violent. The heat evolved is not sufficient for the hydrogen to catch fire. Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g) Calcium starts floating because the bubbles of hydrogen gas formed stick to the surface of the metal. Magnesium does not react with cold water. It reacts with hot water to form magnesium hydroxide and hydrogen. It also starts floating due to the bubbles of hydrogen gas sticking to its surface. Metals like aluminium, iron and zinc do not react either with cold or hot water. But they react with steam to form the metal oxide and hydrogen. 2AI(s) + 3H2O(g) → Al2O3(s) + 3H2(g) 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) Metals such as lead, copper, silver and gold do not react with water at all. (d) Reactions with acids: Metal + Dilute acid → Metal salt + H2 Na reacts violently with dilute HCl: 2Na + 2HCl → 2NaCl + H2 Mg reacts quite rapidly with dilute HCl: Mg + 2HCl → MgCl2 + H2 Zn reacts less rapid than that of Mg: Zn + 2HCl → ZnCl2 + H2 Fe reacts slowly with dilute HCl: Fe + 2HCl → FeCl2 + H2 Nitric acid (HNO3) is strong oxidising agent, so H2 gas is not produced when metal reacts with acids. But magnesium and manganese react with very dil. HNO3 to evolve H2 gas. (e) Reactions with solutions of other metal salts: More reactive metal of can displace less reactive metal from its salt solution. Metal A + Salt solution of B → Salt solution of A + Metal B e.g., Fe + CuSO4 → FeSO4 + Cu (Blue) (Grey) (Greenish-blue) (Reddish-brown) Cu + FeSO4 → No reaction takes place as Cu is less reactive than Fe. 4. The reactivity series of metal: A list of common metals arranged in order of their decreasing reactivity is known as activity series (or reactivity series). 5. Non-metals: All those elements which form negative ions by gaining electrons are called non-metals. They have 4 to 8 electrons in their valence shell (except hydrogen and helium). 6. Physical properties of non-metals: (i) Non-metals are neither ductile nor malleable. (ii) They are either solids or gases except bromine, which is a liquid. (iii) They are bad conductors of heat and electricity except graphite (an allotrope of carbon). (iv) They are non-lustrous and cannot be polished except iodine. (v) They are brittle in nature and have low melting and boiling points except graphite.

METALS AND NON-METALS 47 7. Chemical properties of non-metals: (i) Non-metals form either acidic or neutral oxide with oxygen. Example: Carbon forms acidic oxide like CO2 and hydrogen forms neutral oxide like H2O. (ii) Non-metals do not react with water to evolve H2 gas and they do not react with water. 8. Reaction of metals and non-metals: The reactivity of elements depends upon the tendency to attain a completely filled valence shell. Reaction of sodium and chlorine to form sodium chloride. 9. Properties of Ionic (Electrovalent) compounds: (i) Physical nature: Ionic compounds are solid and hard. These are brittle and break into pieces when pressure is applied. (ii) These compounds have high melting and boiling points. (iii) These are soluble in water and insoluble in organic solvents such as kerosene, petrol. (iv) These conduct electricity in the molten state but not in solid state. 10. Occurrence of metals: The elements or compounds which occur naturally in the earth’s crust, are known as minerals. Those minerals from which metals can be extracted profitably are called ores. All the ores are the minerals but all the minerals are not ores. The impurities of sand and rocky materials present in the ore is called gangue. 11. Extraction of metals: The extraction of metals depends upon the reactivity of metals. Metals of high reactivity are never found as free elements. Metals of medium reactivity are found as oxides, sulphides or carbonates. Metals of low reactivity are found in free state. 12. Enrichment of ores: The removal of gangue particles from the ore prior to the extraction of metal is called enrichment of ores. Techniques like hydraulic washing, froth-floatation, electromagnetic separation and chemical separation are used. 13. Extracting metals low in the activity series: This is done by heating the metals alone as the metals are unreactive. Example: 2HgS(s) + 3O2(g) Heat → 2HgO(s) + 2SO2(g) 2HgO(s) Heat → 2Hg(l) + O2(g) 14. Extracting metals in the middle of activity series: The metals like Fe, Zn, Pb, Cu, etc., are moderately reactive. The metal sulphides and carbonates must be converted to metal oxide. Roasting: The process of converting sulphide ores into oxides by heating strongly in the presence of excess of air. Example: 2ZnS + 3O2 ∆ → 2ZnO + 2SO2 Calcination: The process of converting carbonate ores into oxides by heating them strongly in the limited supply of air. Example: ZnCO3 ∆ → ZnO + CO2 Metal oxides are then reduced to metals by using reducing agents like carbon. Example: ZnO + C → Zn + CO Displacement reactions are also used for reducing metal oxides to metal. Example: 3MnO2 + 4Al → 3Mn + 2Al2O3 + Heat

48 METALS AND NON-METALS 15. Thermit reaction: The reaction of Fe2O3 with Al is known as thermit reaction. This is used to join railway tracks or cracked machine parts. The reaction is highly exothermic. Fe2O3 + 2Al → 2Fe + Al2O3 + Heat 16. Extraction of high reactive metals: The metals which are very reactive usually occur in the form of their oxides. The oxides of such metals cannot be reduced by carbon. These metals are obtained by electrolytic reduction. For example, sodium, magnesium are obtained by electrolysis of their molten chloride. Metals are deposited at cathode and chlorine is liberated at anode. At cathode: Na+ + e– → Na At anode: 2Cl– → Cl2 + 2e– Aluminium is obtained by electrolytic reduction of aluminium oxide. 17. Refining of metals: The process of purification of impure metals. It depends upon: (i) Nature of element and (ii) Nature of impurities present. Most widely method is – ELECTROLYTIC REFINING. In this method, the impure metal is made anode and thin strip of pure metal is made cathode. A solution of metal salt is used an electrolyte. When current is passed, pure metal from anode dissolves into the electrolyte. Equivalent amount from electrolyte get deposited at cathode. 18. Corrosion: The process of slowly eating away of metal due to attack of air, water, etc., on the surface of the metal is called corrosion. Examples: (i) The surface of copper is coated with green layer in moist air due to the formation of basic copper carbonate (CuCO3.Cu(OH)2). (ii) Iron surface gets coated with brown flaky substance called rust on exposure to moist air. This is called rusting of iron. Rust is Fe2O3. xH2O. (iii) Silver tarnishes due to the formation of black coating of silver sulphide (Ag2S). (iv) Aluminium gets coated with thin layer of aluminium oxide (Al2O3). This prevents metal underneath from damage. 19. Prevention of corrosion: Rusting of iron can be prevented by: (i) painting, oiling, greasing iron articles. (ii) galvanising the surface of iron articles (coating with zinc). 20. Alloys: An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. Examples : (i) Pure iron is very soft and easily stretches when hot. But on mixing it with carbon (0.05%), it becomes hard and strong. (ii) Pure gold (24 carat) is very soft. It is alloyed with 2 parts of either copper or silver to make it hard. (iii) Solder, an alloy of lead and tin is used for welding electrical wires together. If one of the metals is mercury, then alloy is called amalgam. Properties of an alloy: (i) It has better properties than the metal in its purest form. (ii) The electrical conductivity of an alloy is less than pure metal. (iii) It has low melting point than its constituents. (iv) It is resistant to corrosion.

METALS AND NON-METALS 49 Some Important Metals and their Ores S.no. Metal Ore Chemical formula 1. Copper Copper CuFeS2 pyrites 2. Zinc Zinc blende ZnS 3. Lead Galena PbS 4. Aluminium Bauxite Al2O3. 2H2O 5. Iron Haematite Fe2O3 6. Mercury Cinnabar HgS 7. Copper Copper Cu2S glance 8. Sodium Rock salt NaCl Some Facts about Metals 1. Most abundant metal — Aluminium 2. Liquid metal — Mercury 3. Lustrous non-metal — Iodine 4. Soft metals — Sodium, Potassium 5. Most ductile metals — Gold, Silver 6. Best conductor metal — Silver 7. Metal kept in Paraffin wax — Lithium 8. Metal kept under kerosene — Sodium, Potassium 9. Heaviest metal — Osmium (Os) 10. Liquid non-metal — Bromine 11. Most electropositive metal — Caesium (Cs)

50 METALS AND NON-METALS ASSIGNMENT (In Class Discussion) Q.1. Which of the following properties is generally not shown by metals? Options: (a) Electrical conductivity (b) Malleability (c) Ductility (d) Dullness Q.2. Generally metals react with acids to give salt and hydrogen gas. Which of the following acids does not give hydrogen gas on reacting with metals (except Mn and Mg)? Options: (a) H2CO3 (b) H2SO4 (c) HNO3 (d) H2CO3 Q.3. Which of the following oxide(s) of iron would be obtained on prolonged reaction of iron with steam? Options: (a) FeO (b) Fe2O3 (c) Fe3O4 (d) Fe2O3 and Fe3O4 Q.4. An element ‘X’ is soft and can be cut with a knife. It is very reactive to air and cannot be kept open in air. It also reacts vigorously with water. The element ‘X’ can be: Options: (a) Na (b) Mg (c) Ca (d) Al Q.5. Which of the following correctly represents the composition of aqua regia? Options: (a) Conc. HCl and Conc. H2SO4 in the ratio 3:1 (b) Conc. HCl and Conc. H2SO4 in the ratio 1:3 (c) Conc. HCl and Conc. HNO3 in the ratio 1:3 (d) Conc. HCl and Conc. HNO3 in the ratio 3:1 Q.6. Which one of the following metals would be displaced from the solution of its salts by other three metals? Options: (a) Mg (b) Ag (c) Zn (d) Cu Q.7. Which of the following metals can melt by taking heat from your palm? Options: (a) Na (b) Ga (c) Cs (d) Both B and C are correct Q.8. Give an example of a metal which: (a) is a liquid at room temperature (b) can be easily cut with a knife (c) is the best conductor of heat (d) is a poor conductor of heat Q.9. What would you observe when zinc is added to a solution of iron(II) sulphate? Write the chemical equation for the reaction that takes place. Q.10. Which of the following listed metals can displace zinc from its salt solution? Give the reason for your answer along with the chemical equation. Copper, Lead, Magnesium, Silver