SCIENCE

(b) A spherical mirror with its reflecting surface on the outside is a __________. (c) An image that can be obtained on a screen is ____________. (d) A plane mirror forms a ____________ image. (e) A convex mirror always forms a ____________ image. 2. Write true for correct and false for the incorrect statements. (a) The sun is a natural source of light. (b) Concave mirrors are used as shaving mirrors. (c) Laws of reflection do not hold for curved mirrors. (d) When the object is kept at the focus of a mirror, an image is formed at infinity. (e) The process of the bending of the light is called reflection. 3. Answer the following questions. (a) Define the refraction of light and state the laws of the refraction of light. (b) Differentiate between a ray and a beam of light. (c) Explain the process of reflection by a plane mirror. 4. Define the terms: (a) Incident ray (b) Concave mirror (c) Focus (d) Refraction index (e) Refraction of light (f) Emergent ray 5. Write the uses of: (a) Plane mirror (b) Convex mirror (c) Concave mirror 6. Write the differences between: (a) Concave mirror and convex mirror (b) Real image and the virtual image 7. Given reasons. (a) A concave lens is called a converging lens. (b) A pencil appears bent when partially immersed into water. (c) The coin at the bottom of the pond seems to be raised up. (d) A concave mirror is used as a reflector in a searchlight. Light 97

(e) A concave mirror is used as a shaving glass. (f) A convex mirror is used as a back rear view in automobiles. (g) Plane mirrors are used to see our face. 8. State the law of refraction of light. 9. When the angle of incidence of a ray is 30°, the corresponding angle of refraction will be 20°. If the mediums were air and water, write with a diagram, from which medium the light emerges and in which medium it enters? 10. Can we use concave mirrors as side view mirrors in vehicles. Why? 11. If a man throws a stone at the place where he sees the fish inside the water. Is it possible that it will hit the fish? Write with a reason. 12. A boy is 3 ft tall. If he jumps into a pond, which appears to be in less depth than 3 ft, will he sink or not? Give a reason. 13. Draw ray diagrams to show the position and the nature of an image formed by the concave mirror when the object is placed: (a) beyond C (b) between C and F (c) at C (d) at infinity (e) between F and P (f) object at F 14. Complete the following ray diagrams lossar - t eet at a sin e ed p int r di erent dire ns - t s a er r a p int t di erent dire ns C onvergent - a e n ra s D ivergent - ray which comes to strike a ray surface Beam - enlarged I ncident a ni ed 98 New Creative Science and Environment Book - 8

U 8 SOUND L earning O utc om es At the end of this unit, students will be able to: ~ define the terms related to sound (velocity, frequency and wavelength). ~ define and differentiate an echo and reverberation and state their effects. Main points to be focused ~ Velocity (V) ~ Types of sound waves ~ Some terms related to sound ~ Reflection of sound ~ Wavelength ( ) ~ Echo ~ Amplitude (a) ~ Reverberation ~ Frequency (f) ~ Time period (T) Introduction We hear sounds from various sources like humans, birds, bells, machines, vehicles, televisions, radios, etc. Sound is a form of energy which produces a sensation of hearing in our ears. It is produced due to the vibration of a body. The objects which produce sound by vibrating are called the sources of the sound. We can also produce a sound by plucking, scratching, rubbing, blowing or shaking different objects. In all these activities, the most important thing is to set the objects vibrating to produce sound. To prove it, let’s do the following activity. Sound 99

A c t iv it y To show that the vibration of an object produces sound Take a shaving blade and fix it to a table or desk as shown in the figure. Bend the upper end of the blade and leave it. The blade begins to vibrate. You can see the vibration of the blade. The vibrating blade also produces a sound. We can hear this sound. Touch the blade carefully with your finger. It stops vibrating. Does it produce any sound now? When the blade was vibrating, the sound was also produced. It proves that sound is produced from the vibrating object. As you touch the blade, the vibration ceases and the sound is not heard. Some terms related to sound Sound is a form of wave. It produces due to disturbance in the medium. Some terms related to sound waves are as follows: avelength ( ) The distance between two compressions or two rarefactions of the sound wave is called wavelength. t is denoted by lamda . ts S unit is metre m . 100 New Creative Science and Environment Book - 8

Amplitude (a) The maximum displacement of a vibrating particle from its mean position is called its amplitude. Its SI unit is metre. The higher the amplitude is the louder will be the sound. Frequency (f) The number of complete vibrations which are produced in per second is called its frequency. Frequency is measured in ert symbol . f a body ma es complete vibrations in one second, we say that its frequency is . The larger units of frequency are KHz and MHz. 6 Time period (T) The time taken by a vibrating body to complete one vibration is called its time period. The time period is always measured in seconds. f a body ta es . s to complete one vibration then we can say that its time period is . s. Thus, from the definition of time period and frequency, we can conclude that, 1 Time period Frequency 1 ∴ Tf Velocity (V) The distance travelled by a sound in per second is called its velocity. Its SI unit is m/s. i.e. ∴T istance travelled by sound s Time ta en t The velocity of sound differs in different mediums. For example, the velocity of sound in air is only m s. ut in iron it is m s. The speed of sound at a particular temperature in various mediums is listed in the table below. State Substance Speed in m / s Solids Liquids Aluminium Steel Iron ater Sea ater distilled Sound 101

Gases Hydrogen Air The speed of light in air is m s and in sound it is m s. ow, can you thin why we hear thunder after some time of lightening only? Solved Numericals i. alculate the speed of sound whose frequency is and wavelength is Solution: . m. iven, frequency .m avelength Speed of sound Here, vf . ms Memory Note ; Sonic boom: when the speed of any object exceeds the speed of the sound then it is said to be travelling at supersonic speed. Such objects produce shock waves in the air that carry a large amount of energy. The air pressure variation associated with this type of shock waves produces very sharp and loud sounds called the sonic boom. It has energy to shatter glass and damage buildings. Questions i. How is sound produced? ii. Why is the flash of lightning seen before we hear thunder? Characteristics of sound wave › Sound is a form of energy. › Sound needs a material medium for its propagation. So, sound is also known as a mechanical wave. It cannot travel through a vacuum. › Sound is a longitudinal wave i.e. the vibration of medium will be along the direction of the propagation of sound. › The velocity of sound is the greatest in solids and the least in gases. › Like the light, sound also shows reflection as well as refraction. 102 New Creative Science and Environment Book - 8

Types of sound waves Sound waves are classified into three types according to their frequency. They are i infrasonic sound ii audible sound and iii ultrasonic sound. (1) Infrasonic sound The sound wave having the frequency less than is called the infrasonic sound. It is produced due to the vibration of very large objects. The sound cannot be detected by the human ear. E.g. sound produced during earthquakes. (2) Audible sound The sound wave having the frequency between and is called the audible sound. It is produced due to the vibration of pipes, strings, drums, human vocal cord, etc. It can be detected by the human ear. (3) ltrasonic sound The sound wave having the frequency greater than is called the ultrasonic sound. It is produced due to the vibration of very small objects. This sound also cannot be detected by the human ear. Animals like dogs, bats, dolphins, etc. can hear this sound. Memory Note owadays, ultrasonic sound or ultrasound is used to detect the condition of babies inside their mother’s womb, and it is also used to form the image of the heart and this technique is known as electrocardiography . Sonar is a device that uses ultrasonic waves to measure the distance, direction and the speed of underwater objects. Transmission of sound The process by which sound travels from one medium to another is called the transmission of sound. As you have already known, sound is a mechanical wave. It cannot travel through a vacuum, and needs a material medium for its propagation. It can travel through all mediums i.e. solid, liquid and gas. However, the speed may be different in each of these mediums. (1) Transmission of sound through solid Sound can travel or propagate through the solids. Its speed is the greatest in solids. (2) Transmission of sound through liquids Sound can travel or propagate through the liquid medium. The speed of sound in liquid is more than in gas and less than solids. Sound 103

(3) Transmission of sound in gases As we know, air is the mixture of different gases. We hear sounds of people talking, birds, vehicles, radios, etc. All of these sounds are transmitted from the sources to our ears due to its transmission through the gases or the air. The speed of sound in the individual gas is also different. But the average speed of sound in the air is about m s. Sound cannot travel in vacuum Sound is a mechanical wave. It needs a material medium for its propagation. Thus, sound cannot travel through a vacuum as there is no material medium. Memory Note ; Rhinoceroses communicate using infrasound of frequency as low as 5 Hz . Question i. Why do astronauts use an electronic medium to talk with each other on the moon? Re ection of sound The phenomenon of the turning back of sound to the previous medium after striking any obstacle on its way of propagation is called the reflection of sound. Sound also shows reflection and obeys all the laws of reflection like the light. A c t iv it y To prove that sound shows reflection Place an alarm clock inside a hollow polyethylene pipe and put it in front of a plane mirror at a certain angle as shown in the figure. Draw a perpendicular line to the plane mirror at the point at which the pipe meets the mirror. ow, put another pipe at the same point and place your ear at its other end. Can you hear the sound? This sound is due to its reflection by the plane mirror. Move the pipe so that you can hear the loudest sound. Mark this point and measure the angle between the normal and the pipe. You will find that the angle of incidence is equal to the angle of reflection. This activity proves that the sound also obeys the laws of reflection. 104 New Creative Science and Environment Book - 8

Consequences of the re ection of sound An echo and a reverberation are formed due to the reflection of sound. Echo If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo. An echo is the repetition of sound caused due to the reflection of the sound by an obstacle at a distance at least 17 m away from the source. Thus, for the formation of an echo, the following conditions need to be fulfilled. › The distance between the source and the obstacle must be at least 17m far. › The sound must have enough energy so that it can travel through such a long distance. › The reflecting surface should have a larger surface area. The minimum distance to hear an echo also depends upon the temperature of air. Echoes may be heard more than once due to successive or multiple reflections. The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land. Ex amples of echo Sound 105

Solved Numericals i. person clapped his hands near a cliff and heard the echo after s. hat is the distance of the cliff from the person if the speed of the sound v is ta en as ms–1? Solution: Given, Speed of sound, v ms–1 Time ta en for hearing the echo, t s vt istance travelled by the sound s m n s the sound has to travel twice the distance between the cliff and the person. ence, the distance between the cliff and the person m Reverberation A sound created in a big hall will persist by the repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection of the sound by an obstacle at the distance less than 17 m that results in the prolongation of the sound is called reverberation. In an auditorium or big hall, the excessive reverberation is highly undesirable. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound absorbent materials like fibreboard, rough plaster, seat coats, etc. But reverberation is beneficial in musical sounds. All the soft objects like clothes, curtains, carpets and the human body are the good absorbers of sound whereas hard and smooth surfaces are the good reflectors of sound. Memory Note 1. Megaphones, horns and musical instruments such as trumpets are all designed to send sound in a particular direction without spreading it in all directions. . einrich udolph ert was born on February in amburg, ermany and educated at the University of Berlin. He laid the foundation for future development of the radio, telephone, telegraph and even television. The SI unit of frequency was named hertz in his honour. 106 New Creative Science and Environment Book - 8

Questions i. Differentiate between an echo and reverberation. ii. Why is an echo not heard in a small room? Answer writing Skills 1. Sound cannot travel through a vacuum. Why?  Sound cannot travel through a vacuum as it is a mechanical wave and needs a material medium for its propagation. 2. Astronauts use an electronic medium to talk with each other on the moon. Why?  Astronauts use an electronic medium to talk with each other because the moon has no atmosphere and sound cannot travel through a vacuum. 3. When a ringing bell is touched, no sound is heard. Why?  When a ringing bell is touched, the vibration is absorbed by our hand and no sound is heard. 4. Generally the ceilings of concert halls, cinema halls and conference halls are curved. Why?  The ceilings of concert halls, cinema halls and conference halls are curved so that sound after reflection reaches all the corners of the hall. 5. An echo is not heard in a small room. Why?  For the formation of an echo, the distance between the source and the obstacle must be at least 17 m. This standard is not fulfilled in a small room. So, an echo is not heard in a small room. 6. Sound is repeated near hills. Why?  Sound is repeated near hills due to the formation of the echo as the reflection of the sound by the nearby hills takes place. 7. Big cinema halls are carpeted and made of some rough materials. Why?  Big cinema halls are carpeted and made of some rough materials to absorb sound and to prevent the formation of an echo and reverberation. So, the dialogues of the film can be heard clearly. SUM M ARY 107 ” Sound is a form of energy which gives the sensation of hearing to our ears. ” Sound needs a material medium for its propagation. ” Sound cannot travel through a vacuum. ” The speed of sound is the highest in solids, higher in liquids and the lowest in gases. ” Sound also shows reflection and refraction like the light. Sound

” The phenomenon of the turning back of sound after striking any obstacle on its way of propagation is called the reflection of sound. ” An echo and reverberation are the two consequences of the reflections of sound. ” An echo is the repetition of sound after it strikes any obstacle which is at least 17m away from the source. ” A reverberation is the prolongation of sound after it gets reflected to any surface less than 17m far from its source. ” An echo is beneficial in musical sounds but harmful in a large auditorium hall, cinema hall, etc. ” Soft objects are good sound absorbers while hard objects are good sound reflectors. Exercise 1. Choose the correct answer. a Sound can travel through i solid only ii liquid only iii gases only iv all of these b Sound cannot travel through i air ii water iii vacuum iv steel c Sound travels in air at with a velocity of about i m s ii ms iii m s iv m s d Hertz is the unit of: i time period ii frequency of sound iii velocity of sound iv loudness of sound e The minimum distance required to produce a distinct echo is: im ii 11m iii m iv 17 m 2. Give reasons. a When a ringing bell is touched, sound ceases. b Astronauts use an electronic medium to talk with each other. c Sound is repeated near hills. d Sound absorbing materials are used in cinema halls. e An echo is not heard in a small room. 3. Differentiate between: a Ultrasonic sound and infrasonic sound b Echo and reverberation 108 New Creative Science and Environment Book - 8

4. Answer the following questions. a Define the terms. i Sound ii Frequency of sound iii Time period iv Wavelength v Echo vi everberation b Explain an experiment to prove that sound is produced due to the vibration of an object. c How can you show that sound also obeys the laws of reflection? d What is an echo? What are the necessary conditions to form an echo? e We hear a louder sound in a newly constructed room than a furnished one. Why? f What can you do to prevent an echo and a reverberation in cinema halls and auditorium? g hat is meant by the frequency of sound Numerical Problems a The thunder is heard after seconds of lightening. Find the distance of clouds from the ground. Suppose that the velocity of sound is m s. (Ans: 6640 m) b n echo returned in s. hat is the distance of the reflecting surface from the source, given that the speed of sound is ms–1 (Ans: 5613 m) c ship sends out ultrasound that returns from the seabed and is detected after . s. f the speed of ultrasound through seawater is m s, what is the distance of the seabed from the ship? (Ans: 2618 m) d A person clapped his hands near a cliff and heard the echo after 4s. What is the distance of the cliff from the person if the speed of the sound v is ta en as ms–1? (Ans: 664 m) e alculate the speed of sound whose frequency is and wavelength is . m. (Ans: 225 m/s) G lo ssa ry A uditorium – the part of a theatre or hall in which the audience sit i ra n ast t and r ve ent r the s i at r n a Fre uen n itudina ave d hi h a pr du e s und the nu er vi ra ns pr du ed per se nd a ave that vi rates in the dire n that it is vin Sound 109

U 9 MAGNETISM L earning O utc om es At the end of this unit, students will be able to: ~ explain the molecular theory of magnetism. ~ define, explain and demonstrate the magnetic induction. ~ say the causes of demagneti ation and methods of conservation of magnetic power. Main points to be focused ~ Demagneti ation ~ Magnetism ~ Protection of the magnetic power of a ~ Molecular theory of magnetism ~ Magnetic field magnet ~ Magneti ation (magnetic induction) ~ Terrestrial magnetism Introduction You must be familiar with a magnet. We use it to attract the magnetic substances like iron, nickel, steel, cobalt, etc. Magnetic substances are the objects which can be attracted by the magnet. The magnet was found at first by the people of China and named it as lodestone or leading stone. It has directive property. So, it was used by navigators to find directions. Magnets may be natural or artificial. Artificial magnets are more powerful and portable than natural magnets. The properties of magnet are called magnetism. Thus,magnetism is a form of energy possessed by a magnet. A magnet has certain properties. They are: › A magnet attracts magnetic substances. › When a magnet is freely suspended, it roughly shows the north-south direction. 110 New Creative Science and Environment Book - 8

› The similar poles of the magnets repel with each other, whereas the opposite poles of the magnets attract to each other. › A magnet has a certain magnetic field around it, up to which it can show its influence. › The poles of a magnet cannot be separated. Molecular theory of magnetism When an iron piece is kept in contact with a magnet then the iron piece becomes a magnet. It can attract other iron pieces towards itself. A magnetic substance can be made a temporary magnet using this method. How could this be possible where a piece of wood cannot be a magnet at all? To understand this, we should know about the molecular structure of a magnet, magnetic substances and the non-magnetic substances. The molecular theory of magnetism helps to answer these questions. The molecular theory of magnetism explains the mechanism of magnetization and demagnetization which can be summarized as: i. All the magnets and magnetic substances are made up of the molecular magnets. They consist of a pair of north and South Pole. So, the poles of a magnet cannot be separated even after breaking it up to its molecules. ii The molecular magnets in a magnet are arranged in straight chains by attracting their opposite poles as shown in the figure. The opposite poles are free only at the poles of the magnet. Hence, the magnetic force is more at its poles than in the middle. But in the magnetic substances, the molecular magnets are arranged in the closed ring structure as shown in the figure. So, they cannot show the magnetic property. But when they are placed in the magnetic field then the molecules get arranged in a straight line and this becomes a magnet. Magnetism 111

De a ne ed su stan e a ne ed su stan e iii hen a magnet is hammered or heated, it loses its magnetic properties because the molecular magnets arrange themselves in a closed chain structure. De a ne ed su stan e Memory Note The two main organs in the human body where the magnetic field produced is significant. They are the heart and the brain. This magnetic field is the basic of obtaining the images of different body parts. This is done using a technique called agnetic esonance maging . Question i. Explain the molecular theory of a magnet. ii. Show the molecular structure of a demagnetized and a magnetized substances. Magnetic field The magnetic field is the area around a magnet up to which it can show its effects. The magnetic field is represented by the magnetic lines of force. The magnetic lines of force have the following properties. 112 New Creative Science and Environment Book - 8

The properties of magnetic lines of force i The magnetic lines of force are the continuous closed curves, which arise from the north pole of the magnet and end at the South Pole. ii The magnetic lines of force never intersect to each other. iii These lines of force are more at the poles than in the middle showing the greater force at the poles. A c t iv it y To show the magnetic field of a magnet lace a bar magnet facing it north and south. ow, place a thick paper over this magnet and scatter iron fillings over the paper. Tap the paper gently and see the arrangement of the iron fillings. The iron fillings arrange themselves as shown in the figure. It is the magnetic field of the magnet. A c t iv it y Drawing magnetic lines of force Take a white sheet of paper and fix it to the drawing N board using thumb pins. Take a small compass and a bar magnet. Mark the boundary of the magnet on the paper. Place the compass near the north pole of the magnet. How does it behave? The north pole of the needle points towards the south pole of the magnet. The point shown by the north pole of the S compass is marked, and this process is repeated till the compass reaches the south pole of the magnet. ow, draw a line joining all these points. This line obtained is the magnetic line of force. epeat this activity to get a number of magnetic lines of force. Observe the above magnetic lines of force and find out whether they intersect each other or not. Memory Note During a classroom demonstration of an experiment to his students, Oersted noticed that a compass needle was deflected when an electric current was switched on nearby. He went on to study this effect. Further, he discovered the connection between electricity and magnetism. Magnetism 113

Magneti ation (magnetic induction) The process of developing the magnetic property to a magnetic substance is called magnetization. It can be done by using the following methods: › By touching a magnetic substance with a magnet. › By rubbing a magnetic substance with a magnet. The process of inducing the magnetic property in a magnetic substance by using a magnet is called magnetic induction. Demagnetization The process of losing the magnetic property of a magnet is called demagnetization. As stated earlier, the demagnetization is due to the disturbance of the open chain structure of the molecular magnet due to some disturbances like hammering, hitting or heating. The main causes of demagnetization are: i egular hammering of the magnet. ii egular dropping it from a height. iii eating of magnet. iv arrying the similar poles using an e ternal force. v eeping the magnet without eepers, etc. Memory Note ; The magnetic substance that is strongly attracted by a magnet is called ferromagnetic substance. For example iron, nickel,cobalt, etc. Protection of the magnetic power of a magnet To protect the magnetic power of a magnet, we should apply the following methods. i. The magnet should not be heated. ii. We should not drop the magnet on the floor. iii. Hammering and rubbing should not be done on the magnet. iv. Magnets should be protected from rusting. v. Magnets should be kept in a magnet keeper. 114 New Creative Science and Environment Book - 8

Terrestrial magnetism The earth itself acts as a big magnet. The magnetic property of the earth is called the terrestrial magnetism. It is said that there is a huge magnet inside the earth whose north pole points towards our earth’s south pole, and whose south pole points towards our earth’s north. But the geographic poles and the poles of the magnet do not lie at the same point. That means there is a gap between the magnetic meridian and the geographic meridian. A bout 2000 km North pole of the earth aa S outh pole of the earth s a net 17° rth p e the earth s magnet S outh pole of the earth The terrestrial magnetism can be verified by the following facts. i. freely suspended magnet rests in the S direction because its pole is attracted by the south pole of the terrestrial magnet, which lies on the geographic north. ii. suspended magnet doesn t point to the e act S direction because the geographic north pole and the magnetic north pole do not coincide. iii. A suspended magnet beyond the equator lies at rest due to the fact, that one pair of the poles lie closer than the other. iv. f the magnetic substance is buried inside the earth pointing S direction for some days, it becomes magnetized. Memory Note ; It is believed that the earth’s magnetic field shows its effect in a region above the earth’s surface known as the magnetosphere. Magnetism 115

Questions i. The poles of a magnet cannot be separated. Why? ii. hen a magnet is freely suspended, it shows the S direction. hy Answer writing Skills 1. The poles of a magnet cannot be separated. Why?  All the magnets are made up of molecular magnets, which consist of a pair of S poles. Thus, if the magnet is even bro en down into its molecules, their poles cannot be separated. 2. A magnet has more force at the poles than at its middle. Why?  The molecular magnets are arranged in an open chain-like structure by attracting their opposite poles. So, the poles are free only at its ends. Hence, the magnet has more force at the poles than at its middle. 3. When a magnet is freely suspended, it roughly shows the N-S direction. Why?  When a magnet is suspended, its poles are attracted by the opposite poles of the earth s magnetism. nd we now that, the pole of terrestrial magnet lies on the geographic south and vice versa. Thus, the magnet shows the S direction. 4. We should not heat a magnet. Why?  As we heat a magnet, the arrangement of molecules of a magnet are disturbed. Therefore, the magnet loses its magnetic property. It is also called demagnetization. 5. What is a magnet keeper?  A magnet keeper is a device where we keep the magnet. It protects the magnetic power of a magnet. SUM M ARY ” The substance which can attract the magnetic substances is called a magnet, and the properties possessed by a magnet is called magnetism. ” The poles of a magnet cannot be separated. ” Like poles of the magnet repel and unlike poles attract each other. ” The molecular theory of magnetism states that, i ll magnets and magnetic substances are made up of molecular magnets having a pair of S poles. ii olecular magnets are arranged in open chain structure in magnets and 116 New Creative Science and Environment Book - 8

closed chain structure in a magnetic substance. ” The process of making a magnet to a magnetic substance by using a magnet is called magnetic induction. ” The process of losing magnetic property of a magnet is called demagnetization. Exercise 1. Fill in the blanks. a The magnetic property of the earth is called __________. b Like poles __________ and unlike poles ____________. c A magnetic field is shown by a _________. d In a magnetic substance, molecular magnets are arranged in ____________. e In a magnet, molecular magnets are arranged in ____________. 2. State True for correct and False for incorrect statements. a The poles of a magnet cannot be separated. b Like poles of the magnets attract and unlike poles repel each other. c Iron is a ferromagnetic substance. d The magnet e actly shows the S direction. e The magnetic force is more in the middle than at the poles. 3. Answer the following questions. a What is magnetism? Write the properties of a magnet. b Define a molecular magnet. State the molecular theory of magnetism. c How can you prove that the earth itself acts as a magnet? d What is a magnetic field? How can we demonstrate it? e Define magnetization and demagnetization. f How can we protect the magnetic power of a magnet? Write down any four points. g List out any four demagnetizing methods. Magnetism 117

4. Differentiate between: a atural and artificial magnets. b Magnetization and demagnetization. 5. Give reasons. a The poles of a magnet cannot be separated. b A magnet has more force at the poles than in the middle. c Magnetic substances do not show the magnetic properties though they are also made up of molecular magnets. d freely suspended magnet shows roughly the S direction. e A magnet should not be heated and hammered. 6. Study the diagram and answer the following questions. ( a) ( b) a Which one is a magnet and which one is a magnetic substance? b Why do magnetic substances not show their magnetic properties? G lo ssa ry t push a a eas t arr r p a e t p a e R epel t have the sa e p si n in spa e Portable - man-made C oincide Ar ia 118 New Creative Science and Environment Book - 8

U 10 ELECTRICITY L earning O utc om es At the end of this unit, students will be able to: ~ explain the general structure and application of a simple cell and a dry cell. ~ define the house wiring system and its devices. ~ state the definition of a fuse and a MCB (Miniature Circuit Breaker). Main points to be focused ~ Combination of cells ~ Static electricity ~ House wiring system ~ Current electricity ~ Some electrical devices ~ Electric charge ~ Fuse ~ Simple cell ~ MCB ~ Dry cell A luxurious life is the gift of the current electricity. In most part of our life, we use electric currents. It can be used for cooking, heating and lighting. It is also used in refrigerators, vehicles, radios, TVs, mobiles, electromagnets, etc. Electricity is also used for running large machines and equipment in factories, industries, the press, hospitals, etc. It means currents can be used for multi purposes. Electricity Electricity is a form of energy, which can be produced due to the change in the number of electrons in a body or a flow of electrons through a conductor. On the basis of how it is produced, there are two types of electricity. They are: i Static electricity ii urrent electricity i) Static electricity Each and every substance is made up of atoms. All the atoms contain an equal number Electricity 119

of positively charged protons and negatively charged electrons. Therefore, all the atoms are electrically neutral. It also shows that every matter is electrically neutral. When two bodies are rubbed, the electrons flow from one body to another making them positively and negatively charged. t means electricity can be developed by rubbing friction the two bodies each other. This produced electricity is called static electricity. Thus, the electricity produced due to the change in the number of electrons in a body is called static electricity. It is produced in the insulators and cannot be transferred from one point to another. For example, when we rub a plastic pen or a plastic comb on our head and bring near the pieces of paper, the C omb papers get attracted towards the pen. It is because static electricity is produced in the pen due to the Bits of paper change in the number of electrons during its rubbing. In this case, the substance which gains electrons is negatively charged and the substance which loses electrons is positively charged. Memory Note ; Static electricity causes lightening in the sky. ii) Current electricity In any substance, the pole accumulating more electrons is negatively charged and the pole accumulating fewer electrons is positively charged. As these positive and negative terminals are connected by using a conducting wire, there Current electricity is a flow of electrons. This flow of electrons is called current electricity. Thus, the electricity produced due to the flow of electrons through a conductor is called current electricity. It is a useful form of energy, and can easily be changed into other different forms. Memory Note ; Electricity is the flow of charge (electrons) in per unit time. 120 New Creative Science and Environment Book - 8

A c t iv it y Pen Bits of paper To make a charged body Take a plastic pen or a plastic comb and rub it on your head. Bring it near the dust particles of your classroom. You will see that the dust particles are attracted towards the pen. It is because when the pen is rubbed, the number of electrons in the pen gets changed and a charge is developed. Thus, it attracts the oppositely charged dust particles towards it. Electric charge Matter is made up of atoms. An atom is electrically neutral due to the equal number of protons and electrons. But when the atoms gain or lose the electrons in any way, then the charge is developed. If the electrons are gained by a body, it gains negative charge and if the electrons are lost by the body then it bears the positive charge. Thus, the electrical property of a particle is called an electric charge. The similar charges repel and the dissimilar charges are attract to each other. Question i. How is charge produced in a body? Sources of electricity The objects which can produce electricity are called the sources of electricity. For example, a cell, dynamo, generators, etc. Cell It is a source of electricity which changes the chemical energy into the electrical energy. There are two types of cells. They are: rimary cell Secondary cell (A) Primary cell The cell which cannot be recharged or reused is a called primary cell. It is also of two types. They are: i Simple cell ii ry cell i. Simple cell The simple cell is simple in structure. It consists of a beaker filled with dilute sulphuric acid, two plates one is inc plate and another is copper plate , a conducting wire and a bulb or galvanometer . Electricity 121

The plates are dipped into the acid. They are connected with a conducting wire with a galvanometer or a bulb as shown in the figure. Due to the chemical reaction, the copper plate becomes positively charged, and a zinc plate becomes negatively charged. ow, the electrons flow from the zinc plate to the copper plate by producing electricity. It is shown by the deflection in the galvanometer. This type of cell is also known as galvanic cell. Working mechanism of a simple cell The dilute H SO4 ionizes into its opposite ions as, H SO4 + + SO4_ _ H+ ions are attracted towards the copper plate and SO4_ _ ions are attracted towards the zinc plate. At copper plate H+ ions move towards the copper plate and form the molecules of hydrogen by gaining electrons from it i.e. + e– Here, the copper plate loses electrons. So, it becomes positively charged. At zinc plate SO4– – ions move towards the zinc plate and form ZnSO4 by reacting with zinc as, ne Zn++ Zn++ + SO4 ZnSO4 Since, the electrons are produced on the zinc plate, it becomes negatively charged. Due to the flow of these free electrons from zinc to copper i.e. from the negative terminal to the positive terminal, current electricity is generated, which is shown by the deflection in galvanometer. Memory Note ; The simple cell was invented by an Italian scientist named Volta. Defects of the simple cell The simple cell is very simple in construction. It is not possible to produce current for a long time from this cell. This is due to the following two defects. 122 New Creative Science and Environment Book - 8

i) Polari ation It is a defect produced due to the formation of the hydrogen layer on the copper plate. As, we know, hydrogen ions (H+ move towards a copper plate and gain electrons from there to form H molecule. This hydrogen layer is the insulator and prevents the flow of electrons through it. Thus, current doesn’t flow at all. To prevent this defect, a concentrated solution of potassium dichromate (K Cr O7 is added to the solution. This K Cr O7 reacts with hydrogen and forms water molecule. Thus, it removes polarization. So, K Cr O7 is also known as a depolarizer. The other depolarizers are copper sulphate (CuSO4 , manganese dio ide n , etc. ii) Local action t is due to the impurities li e copper u , carbon , iron Fe , etc. present in the zinc plate. Due to these impurities, the local current flows inside the cell acting zinc as the negative terminal and the impurities as the positive terminal. Thus, the internal resistance of the cell increases and the cell becomes defected. Due to the local action, the zinc plate becomes thin and weak. To remove local action, pure or amalgamated zinc must be used. ses of a simple cell A simple cell is used to generate small scale of electricity. Whenever we need electricity, we can set up a simple cell to produce it. In this cell, we use liquid acid; therefore it is difficult to carry it from one place to another. Question i. What is the role of MnO in a simple cell? ii. What happens when we bring an impure zinc plate while making a simple cell? b) Dry cell The simple cell contains a liquid i.e. H SO4. This liquid makes the cell inconvenient and importable. Thus, to remove this defect, it is improved to form a dry cell. The dry cell consists of a paste instead of liquid. It consists of a muslin bag with a carbon rod in it. The bag is filled with a mixture of manganese dioxide and carbon powder. The carbon rod consists of a brass cap at its upper end. A cardboard is placed at the bottom of the zinc can and the muslin bag is kept inside the zinc can. The gap between the bag and the zinc vessel is filled with a paste of ammonium chloride. Then the open end of the can is sealed with wax. Electricity 123

si ve terminal r te ve casing e tr te C arbon + M nO 2 paste Z inc C arbon rod A dr e e a ve ter- minal ses of a dry cell The dry cell contains of a paste instead of liquid. So, it is easily portable. It can be made in different sizes and shapes. It can be used in radios, torch lights, toys, TV remote, etc. This cell contains a paste of manganese dioxide (MnO . So, polari ation is not possible. But local action is possible as it contains zinc. Therefore, without using it, if we keep a cell for a long time its life time decreases due to the local action. A c t iv it y Take an old and damaged dry cell and cut it to open. Observe inside and try to find the chemicals used in it. B) Secondary cell The cell which can be recharged and reused is called a secondary cell. For example, lead- acid battery. t was invented by aston lanet in . Question i. We should not keep a dry cell for a long time. Why? ii. What is the main difference between primary and secondary cells? Combination of cells We need to use more than one cell for our various purposes. They can be arranged in different ways. This arrangement of cells is called combination of cells. A group of cells is called battery. There are three types of combination of cells. They are: i Series combination ii arallel combination iii i ed combination 124 New Creative Science and Environment Book - 8

(i) Series combination of cells Series combination of cells The type of combination of cells in which the positive terminal of one cell is connected to the negative terminal of another cell is called the series combination. In this type of combination of cells, if a bulb is connected to the circuit, its brightness increases as the number of cells increases. But the cells do not last for a long time. (ii) Parallel combination of cells The type of combination of cells, in Parallel combination of cells which all the positive terminals of the cells are connected to one point, and all the negative terminals of the cells are connected to another point is called the parallel combination. In this combination, the brightness of the bulb does not change along with the increase in the number of cells. But the bulb can light for a long period of time. (iii) Mixed combination of cells A mixed combination is the mixture of both the series and parallel combination of the cells. Thus, from the mixed combination, we can get both the benefits of series and parallel combination of the cells. + -+ - + -+ - -+ - + -+ - + + + - -+ - + - Mixed combination of cells A c t iv it y Take three dry cells and connect them in series combination. Observe the brightness of the bulb. ow, connect these cells again in parallel combination and observe the brightness of the bulb. Compare the brightness of the bulb in both the cases. Electricity 125

House Wiring System Current electricity can be produced in the electric power plants by using the dynamo or generator. The power plants are run by using different raw materials such as running water, coal, petroleum products, solar energy, nuclear fuel, etc. Generally, the electric power plants are established far away from the human settlement and the industries. So, electric current is carried through high tension transmission line to the sub-stations or distribution centres. The voltage of electric current in the transmission line is very high. It is converted to less voltage using step down transformers. With the help of local transmission lines, the current electricity is passed to the separate houses. The local distribution line is connected to a main fuse before connecting it to the meter. ow, the line is connected to the main switch or the main circuit brea er. The line coming from the main switch is connected to the distribution board. From the distribution board, different electric loads are connected. n the household wiring, there are three different wires. amely phase wire or live wire , neutral wire and the earthing wire. The phase wire or live wire carries current to the electrical devices. The neutral wire carries current back to the power station. Similarly, the earthing wire is connected to the covering of the main switch box and then passed to the earth. The main function of the earthing is to pass leakage current and excessive current to the ground and to prevent it from an electric shock. The different kinds of electric devices such as bulbs, tube lights, heaters, TVs, refrigerators, etc. are connected to the live wire. These devices are called electric loads. The above electric loads are connected in the parallel combination. Therefore, they can run separately. House wring system Some Electrical Devices Electricity is an important source of energy. It is used to run different types of electrical devices. These devices change electrical energy into other forms of energy. Those devices which run by using electrical energy are called electrical devices. Some electrical devices are briefly explained in this unit. 126 New Creative Science and Environment Book - 8

(a) Electric lamp or filament bulb An electric lamp or a filament bulb is the F ilament cheapest lighting device. It is used to convert Supp r n ire electrical energy into light energy. It contains a tungsten filament covered with a glass shield. Nitrogen gas This filament turns about percentage of the electrical energy into light energy. Its estimated G lass stem life time is about hours. Thick wire A u iniu verin Pin Terminals elec t ric bulb (b) Tube light or fluorescent lamp S tarter The tube light or fluorescent lamp er ur vap ur is an eye suitable lighting device. It is used to convert electrical F luorescent power Tube light energy into light energy. It has a hollow glass rod filled with mercury vapour. This mercury Electrode vapour produces UV rays. The A 220v inner wall of the glass rod is C hoke coil Fluorescent lamp pasted with a fluorescent power. This fluorescent powder turns UV rays into the visible rays. The tube light turns about percentage of electrical energy into light energy and its estimated life time is about hours. (c) eater The electric heater is a heating device. It changes electrical energy into heat energy. It has a wire made up of an alloy of nickel and chromium called nichrome. It resists the flow of electric current and produces heat. It is used to make an electric heater, electric iron, rice cooker, electric kettle, etc. Electricity 127

(d) Electric bell Electric bell produces sound as we C ontact G ong press the switch. It is used in an I ron strip office, home, vehicles, etc. It is an electrical device which runs by the S witch application of electricity. As we press the switch, electricity flows and Ba er produces a temporary magnet called electromagnet. With the help of this G ong magnet, a hammer hits the gong to produce a sound. C oil of electromagnet (e) Radio and Television The radio and television are important electrical devices. They run by using electricity or a battery. They are used to get information as well as for the entertainment. They are the icons of modern civilized society. (f) Telephone and Mobile The telephone and Mobile also run by using electricity. They are very important two ways means of communication and information. (g) Computer We can say that it is the age of computer. We use computers in various purposes. It also runs with the help of electricity. It is used in the field of information and communication. It helps to send e-mail on the internet. Fuse A fuse is a safety device. It is made up of an alloy of tin and lead. It has a low melting point with high resistance. It is connected to the phase wire or live wire. It burns if a large amount of current tries to pass through it. As a result, it prevents the electrical devices from being damaged. In different houses, floors, and rooms, we use different kinds of fuses of different rating. Before selecting the appropriate fuse at a particular place, the total amount current must be calculated. 128 New Creative Science and Environment Book - 8

F use wire F use wire Porcelain fuse Cartridge fuse Symbol of fuse MCB (Miniature Circuit Breaker) The MCB and fuse are the protective devices. They protect the overflowing and over loading of current. MCB is a developed form of a fuse. In the situation of the short circuiting, the MCB switch off the circuit in a very short interval of time milli seconds . hen we repair the errors and faults, the is switched on again. Therefore, the MCB avoids the re-connection of the fuse wire as we do in a fuse. Answer writing Skills 1. There is no polarization in dry a cell. Why?  In a dry cell, a paste of manganese dioxide (MnO is used as a depolari er. nside the cell, manganese dioxide reacts with hydrogen to form water. So, there is no deposition of hydrogen gas for the polarization. 2. In series combination, a lamp glows more brightly when the number of cells goes on increasing. Why?  As the number of cells goes on increasing in series combination, the current also increases and it increases the brightness of the bulb. 3. The two glass rods rubbed with fur, repel each other. Why?  When the glass rods are rubbed with a fur then similar charges are developed in these glass rods. Thus, they repel each other. 4. We get an electric shock as we touch a naked electric wire. Why?  We get an electric shock as we touch a naked electric wire. It is because our body acts as a good conductor and the current can pass through it causing an electric shock. 5. When we remove polyester clothes, a ‘tick-tick’ sound is heard with small flash of light. Why? Electricity 129

 When we remove polyester clothes, electrons from our body are transferred to the clothes. Due to this, charges are developed. So, we hear ‘tick-tick’ sounds with small flashes of light. SUM M ARY ” Electricity is a form of energy which can be produced due to the change in the number of electrons in a body or a flow of electrons through a conductor. ” The electricity produced due to the change in the number of electrons in a body is called the static electricity. ” The electricity produced due to the flow of electrons through a conductor is called the current electricity. ” The electrical property of a particle is called electric charge. ” The objects which can produce electricity are called the sources of electricity. ” The cell is a source of electricity which changes the chemical energy into the electrical energy. ” The cell which cannot be recharged or reused is called a primary cell. ” The simple cell was invented by an Italian scientist named Volta. ” The cell which can be recharged and reused is called a secondary cell. ” The arrangement of cells is called the combination of cells. ” A group of cells is called battery. ” The type of combination of cells in which the positive terminal of one cell is connected to the negative terminal of another cell is called the series combination. ” The type of combination of cells in which all the positive terminals of the cells are connected at one point, and all the negative terminals of the cells are connected at another point is called the parallel combination. ” A mixed combination is the mixture of both series and parallel combination of the cells. ” The phase wire or live wire carries current to the electrical devices. ” The neutral wire carries current back to the power station. ” The earthing wire is connected to the covering of the main switch box and then passed to the earth. ” A fuse is a safety device. It is made up of an alloy of tin and lead. ” An MCB is a developed form of a fuse. 130 New Creative Science and Environment Book - 8

Exercise 1. Fill in the blanks using suitable words. a ________ is a developed form of a fuse. b ________ converts the chemical energy into the electrical energy. c The _________ wire carries current to electrical devices. d The cell _________ can be recharged and reused. e The brightness of the bulb increases in _________ combination of the cells. f Brightness of the bulb remains the same in _________ combination of the cells. 2. Write true for the correct and false for the incorrect statements. a An electric lamp glows for a long time in the series combination. b A primary cell cannot be recharge. c A fuse is better than an MCB. d A live wire carries current from the power house to the loads. e In the house wiring, loads are connected in series. f A group of cells is called battery. 3. Differentiate between: a Static electricity and current electricity. b Simple cell and dry cell. c Primary cell and secondary cell. d Series combination and parallel combination of cells. 4. Give reasons. a A simple cell is importable. b A dry cell does not have polarization. c An MCB is better than a fuse. d Glass rods rubbed with fur repel each other. e Electric lamp glows more brightly in a series combination. 5. Answer the following questions. a How is a simple cell constructed? Describe it with a diagram. b Describe the structure of a dry cell with the help of a diagram. Electricity 131

c What is the role of MnO in a dry cell? d What are local action and polarization? How do they make the cell defective? e Describe the series and parallel combination of cells with the help of a diagram. f Write down the advantages of a dry cell and a simple cell. 6. Study the given figure and answer the following questions: a What type of combination of cells is shown in the given figures? b Write down an advantage of this type of combination of cells. c Which combination of cells do you use if you have to glow a bulb for a longer time? G lo ssa ry t push a a th eas t arr r p a e t p a e urrent e e tri it in R epel a t pe a ne transparent n Portable an instru ent used t dete t the M uslin a circuit a van eter 132 New Creative Science and Environment Book - 8

Unit 11: Matter 134 CHEMISTRY Unit 12: Mixture 152 Unit 13: Metals and Non-Metals 159 Unit 14: Acid, Base and Salt 167 Unit 15: Some Useful Chemicals 179 133

U 11 MATTER L earning O utc om es At the end of this unit, students will be able to: ~ state the characteristics of electrons, protons and neutrons after showing the atomic structure. ~ give the simple definition of the periodic table. ~ define the valency and find out the valency of first elements after observing their electronic configuration. ~ define atomic weight and atomic number, and calculate the number of electrons, protons and neutrons based on atomic weight and atomic number. ~ define molecular weight and its calculation. ~ define chemical reaction and chemical equation and write some simple reactions in words and formula. Main points to be focused ~ Duplet rule and octet rule ~ Atom ~ Radicals ~ Structure of an atom ~ Molecular weight ~ Molecules ~ Classification of elements ~ Atomic number and atomic weight ~ Chemical reaction ~ Electronic configuration ~ Reactants and products ~ Valency Introduction Each and everything is made up of matter. Thus, around us there is nothing but only matter. To study this matter we have a separate branch in science which is called chemistry. The branch of main science which deals with matter, its composition, properties and its relation with energy is called chemistry. The surrounding substances which we see, observe and used are not the same type of matter. Some of them are pure matter and the rest are impure matter. Silver coin, Iron rods, golden coins, aluminum pots, glucose powder, common salt, etc. are the pure matter whereas food, clothes, soil, etc. are the impure matter. A brief idea about the division of matter is given below: 134 New Creative Science and Environment Book - 8

a er ure su stan e pure su stan e e ent i ture p und eter ene us i ture ene us i ture Memory Note ; Matter is anything which has weight and occupies space. Atom Each and every element is made up of very small and tiny particles. These particles are indivisible and take part in chemical reaction without division. These ultimate particles of the element are called atoms. The smallest particle of an element which is indivisible and takes part is chemical reaction as a whole without division is called an atom. Atoms which are present in an element are all similar and atoms which are present in different elements are dissimilar to each other. For example, all the atoms present in an iron rod are the same whereas the atoms of an iron rod and atoms of an aluminium rod are different to each other in all respect of structure and appearance. Therefore, there are 118 different atoms of the same number of elements. Structure of an atom e e tr n As we have discussed already that, an atom is the ultimate particle of an element. It is the smallest and indivisible nu eus particle of the element. It is r t n and neutr n such a tiny structure that she it cannot be seen by the naked eyes. Its diameter is Stru ture an at approximately 10–10 m. It is made up of three fundamental particles called electrons, protons and neutrons. Protons and neutrons are situated within a very small volume called the nucleus, whereas electrons are revolving outside the nucleus making the circular path called shells. Matter 135

a. Electrons They are negatively charged sub-atomic particles which are present outside the nucleus in different shells. They have mass equal to 1 th mass of proton. It 1837 the is so small that its mass is not considered while calculating the atomic mass of an atom. It is denoted as e–. An atom consists of an equal number of protons and electrons. So, an atom is electrically neutral. b. Protons They are positively charged sub-atomic particles which are present inside the nucleus of an atom. They have mass equal to the hydrogen atom. They are denoted by the symbol p+ . c. Neutrons They are chargeless sub-atomic particles which are present in the nucleus of an atom. They have mass equal to hydrogen atom. They are denotedby n°. A brief summary of sub-atomic particles is given in the table: S.N. Name of particles Symbol Position Mass in (a.m.u) Charge in e.s.u. 1. Electron 2. Proton e– Shell 1 amu –1 esu 3. Neutron p+ Nucleus 1837 + 1 esu n° Nucleus 0 esu 1 amu 1 amu Molecules We can divide a compound into small particles. The smallest particle of a compound which can exist independently is called a molecule. A molecule also cannot be seen by our naked eyes. The smallest particles of a compound which can exist independently and contain all the properties of the compound are called molecules. Molecules of a matter are all alike but molecules of different matter are different. In some molecules, they have one atom like helium, neon, argon, krypton, xenon, radon, etc. If a molecule contains more than one atom, they may be similar or dissimilar. For example H2, O2, N2, Cl2, etc. Here the same types of two atoms are present in each molecule. CaCO3 , NH3 , CH4, HCl, etc. have different types of atoms. 136 New Creative Science and Environment Book - 8

Atomic number In an atom, there are electrons protons and neutrons. The number of protons which are present in the nucleus of an atom is called an atomic number. The total number of protons which are present in the nucleus of an atom is called an atomic number. In neutral condition, the number of electrons and protons are equal. Atomic mass (or Atomic weight) As we know that, an atom has electrons, protons and neutrons. Protons and neutrons are present in the nucleus of an atom. This sum of protons and neutrons is called atomic mass. Atomic mass or atomic weight is the sum of protons and neutrons that are present in the nucleus of an atom. Memory Note ; The weight of the nucleus is also called atomic weight. Questions (i) Which are the fundamental particles of an atom? Describe them in brief. (ii) Define atom, molecule and matter with example. (iii) What is the main difference between an atom and a molecule? (iv) Give a suitable reason why an atom is a neutral particle. Electronic configuration We know that electrons are present outside a nucleus in different shells. They are not present at rest but are revolving around the nucleus with a high speed. These electrons are present in a systematic way in different shells. Electronic configuration is the systematic distribution of electrons in different shells outside the nucleus of an atom. All the electrons of an atom are not present in one shell but they are distributed under a certain rule. This rule is also known as Bohr and Bury rule. It has the following points. 1. For each and every shell, the maximum number of electrons is determined by 2n2 formula. Where ‘n’ is the number of shells. For example: Matter 137

Total number of electrons in K shell = 2n2 = 2 × 12 = 2 Total number of electrons in L shell = 2n2 = 2 × 22 = 8 Total number of electrons in M shell = 2n2 = 2 × 32 = 18 2. The maximum number of electrons is not more than 8 and 18 in the outermost and second outermost shell of an atom respectively. 3. It is not compulsory to use 2n2 formula everywhere, but a new shell can be started as there are 8 electrons in the last shell. 4. As we move out from the nucleus, the level of energy increases. It means, in first shell there is least amount of energy and in last shell there is more energy. Electronic configuration of first 20 elements At. At. No. of electrons in different orbits S.N. Elements Symbol Protons Elements Neutrons K LM N num mass 1. Hydrogen H1 1 1 0 11 2. Helium He 2 2 2 2 42 3. Lithium Li 3 3 3 4 7 21 4. Beryllium Be 4 4 4 5 9 22 5. Boron B5 5 5 6 11 2 3 6. Carbon C6 6 6 6 12 2 4 7. Nitrogen N7 7 7 7 14 2 6 8. Oxygen O8 8 8 8 16 2 6 9. Fluorine Fe 9 9 9 10 19 2 7 10. Neon Ne 10 10 10 10 20 2 8 11. Sodium Na 11 11 11 12 23 2 8 1 12. Magnesium Mg 12 12 12 12 24 2 8 2 13. Aluminium Al 13 13 13 14 27 2 8 3 14. Silicon Si 14 14 14 14 28 2 8 4 15. Phosphorus P 15 15 15 16 32 2 8 5 16. Sulphur S 16 16 16 16 32 2 8 6 17. Chlorine Cl 17 17 17 18 35 2 8 7 18. Argon Ar 18 18 18 22 40 2 8 8 19. Potassium K 19 19 19 20 39 2 8 8 1 20. Calcium Ca 20 20 20 20 40 2 8 8 2 138 New Creative Science and Environment Book - 8

Questions 1. What is 2n2 formula? Explain it with two examples. 2. What is electronic configuration and show the electronic configuration of Na, Cl, K and Ca in different shall with atomic structure. Atomic structure of first 20 elements are given below: Matter 139

Valence shell and valence electrons From the given atomic structure, it is clear that the last shell of an atom is a valence shell and the total number of electrons present in this shell is called valence electrons. a en e e e tr ns a en e she The outermost shell of an atom is called a valence shell and the total number of electrons present in a valance shell are called valence electrons. Valency In our surroundings, we can observe various types of chemical substances. These substances contain molecules. Each molecule is made up of different types of elements. To make a molecule, atoms of the element possess some force of attraction called valency. The combining, capacity of an element or the radical with other elements or radical to form a stable molecule is called valency. Nowadays, a new definition of valency has been given. It is based on the number of electrons transfer or shared during the chemical reaction. The total number of electrons which are gained or lost or shared by an atom during the chemical reaction is called its valency or valency number. From the above definition, it is clear that the valency of potassium is one because it loses one electron and the valency number of nitrogen is three because it gains three electrons. Elements having valency one are called monovalent, two are bivalent, three are trivalent and four are called tetra valent. Memory Note ; In radical the total number of charges present in it indicates the valency. E lem en t H e L i Be B C N O F e Na A l S i P S C l Ar K C a V a len c y N o . 10 1 2 3 4 3 2 1 0 1 2 3 4 3 2 1 0 12 ; Inert gases like He, N e, Ar, Kr, X e and Rn have z ero valency. ; Some elements show variable valency like ; Cuprous (Cu+ ) = 1 ; Ferrous (Fe+ + ) = 2 ; Cupric (u+ + ) = 2 Ferric (Fe+ + + ) = 3 140 New Creative Science and Environment Book - 8

Questions 1 Define a valence shell and valence electrons with examples. 2. What this meant by valency? Why do some elements show variable valency? 3. Sodium and chlorine both have 1 and 7 valence electrons respectively but they have valency one. Why? Inert gases There are a total of six elements which have 8 electrons in the valence shell (except helium) and do not participate in the chemical reaction due to zero valency (combining capacity). These elements are called inert gases. Those elements which have 8 electrons in their valence shell (except helium which has two electrons) and are inactive in chemical combinations are called inert gases. S.N. Inert gas Symbol Atomic Electronic configuration number L M N OP 1 Helium He 2 K 10 2 8 2 Neon Ne 18 88 36 2 8 18 8 3 Argon Ar 54 8 18 18 8 86 2 8 18 32 18 8 4 Krypton Kr 2 2 5 Xenon Xe 2 6 Radon Rn Duplet and Duplet rule The presence of two electrons in the K-shell (which must be the outermost shell) of an atom is called duplet. Helium is the single element which has such condition. Due to the presence of two electrons in its last shell, it is stable. The tendency of an atom to make two electrons in their K-shell (which must be the outermost shell) is called the duplet rule. Octet and Octet rule There is a total of five elements which have 8 electrons in their valence shell. This type of arrangement is called octet. Thus, the arrangement of 8 electrons in the last shell of an atom is called octet. The five elements which have 8 electrons in their valence shell are in stable condition. So, except inert gases, other elements try to maintain 8 electrons in their last shell by the gaining or losing of electrons is called the octet rule. Matter 141

Molecular formula H2 , O2 , CO2 , NH3 , etc. are the examples of some molecular formula. Here, each molecule is made up of two or more same or different atoms. Thus, the symbol of one molecule of a substance is called its molecular formula. Memory Note ; Inert gases are monoatomic molecules. ; H2 denotes one molecule of hydrogen and 2H denotes two atoms of hydrogen. Radicals To write the molecular formula of a compound, first we should know the symbol of radicals with their valency. Therefore, radicals are the atoms or group of atoms which bear charge on them. According to the type of charge they bear, radicals are classified as electropositive radicals and electronegative radicals. The radicals with their valency are listed below: 1. Electro-positive radicals or basic radicals Racicla Bivalent Trivalent Tetravalent Radical Symbol Valency Radical Symbol Valency Radical Symbol Valency Radical Symbol Valency Hydrogen = H+ = 1 Beryllium = Be++ = 2 Boron = B+++ = 3 Stannic = Sn++++ = 4 Lithium = Li+ = 1 Magnesium = Mg++ = 2 Aluminium = Al+++ = 3 Plumbic = Pb++++ = 4 Sodium = Na+ = 1 Calcium = Ca++ = 2 Ferric = Fe+++ = 3 Potassium = K+ = 1 Strontium = Sr++ = 2 Auric = Au+++ = 3 Rubidium = Rb+ = 1 Barium = Ba++ = 2 Cesium = Cs+ = 1 Cupric = Cu++ = 2 Cuprous = Cu+ = 1 Mercuric = Hg++ = 2 Silver = Ag+ = 1 Ferrous = Fe++ = 2 Mercurous = Hg+ = 1 Stannous = Sn++ = 2 Aurous = Au+ = 1 Zinc = Zn++ = 2 Ammonium =NH4+ = 1 2. Electronegative radicals or acid radicals n va ent Biva ent riva ent Radical Symbol Valency Radical Symbol Valency Radical Symbol Valency Fluoride = F– =1 Oxide Nitride Chloride = Cl– =1 Sulphide = O– – =2 Phosphate = N– – – =3 Bromide = Br– =1 Carbonate Iodide = I– =1 Sulphate = S– – =2 = PO4– – – = 3 Cyanide = CN– =1 Sulphite Hydroxide = OH– =1 Silicate = CO3– – = 2 = SO4– – = 2 = SO3– – = 2 = SiO3– – = 2 142 New Creative Science and Environment Book - 8

Methods of writing molecular formula The following steps should be followed while writing the molecular formula. Methods of writing molecular formula Following steps should be followed while writing the molecular formula. 1. Write symbols of positive and negative radicals side by side, with their valencies. For example: Al3 Cl1 2. Exchange the valency of these radicals. For example: Al3 C 1 l 13 3. Combine these radicals. For example: Al3 C 1 l 13 AlCl3 4. When there is the same valency it is cancelled. For example: C 2a C O2 3 11 5. When radicals are made by two or more different atoms, they are enclosed within bracket. For example, Aluminium hydroxide Al(OH)3 6. While writing the molecular formula, all the letters should be in small letter except first. For example, Aluminium chloride. Some examples of molecular formula 1. Sodium carbonate 2. Hydrogen phosphate N 1a C O32 H1 P O43 21 31 Na2CO3 H3PO4 3. Calcium carbonate 4. Aluminium hydroxide C 2a C O32 Al3 O H1 1 1 13 CaCO3 Al(OH)3 Matter 143

Information obtained from the molecular formula i. The molecular formula is a symbol for one molecule of the substance. ii. It shows the number of atoms of each element present in the molecule. iii. The molecular weight of each molecule can be calculated from the molecular formula. iv. The percentage composition of each element can be calculated from the molecular formula. Molecular weight Molecular weight is calculated by adding the atomic weight of all atoms of the molecule. For example: Molecular weight of HCl = 1×H + 1×Cl Molecular weight of KClO3 = 1 + 35 = 36 Molecular weight of H3PO4 = 1×K + 1×Cl + 3×O = 39 + 35 + 3 × 16 = 122 Classification of elements = 3×H + 1×P+ 4×O = 3×1 + 31 + 4×16 = 98 Since, there are many elements discovered so far. It is difficult to study these elements at their individual level. Thus, there is a need to categorize these elements. However, many such methods are developed. The first successful attempt was made by a Russian scientist Dmitri Mendeleev in 1969 A.D. He made a systematic arrangement or classification of the elements on the basis of their atomic weight in the tabular form, which is called Mendeleev’s periodic table. This was the first scientific table ever made. Mendeleev’s periodic table was based on the periodic law which states that, the physical and chemical properties of the elements are the periodic functions of their atomic weights. It means that, if the elements are arranged in the increasing order of their atomic weight, after a certain interval, the properties of the elements become the same. The elements with similar characteristics are arranged in one group. The Mendeleev’s periodic table consists of the horizontal rows called periods and vertical columns called groups. 144 New Creative Science and Environment Book - 8

A part of Mendeleev’s periodic table is given below: G roup I G roup I I G roup I II G Grouropup V IV G roupG roVuI p VGII roup VIII P eriod 1 H P eriod 2 P eriod 3 Li Be BC N O F P eriod 4 Na M g Al S i PS Cl K Ca 1* Ti V Cr Mn Fe 2* 3* As Se Cu Zn Br Co Ni P eriod 5 Rb S r Y Zr Nb Mo 4* Ru In l Rh Ag C d Sn Sb Te Pd P eriod 6 Cs Ba La H f Ta W Re Os Ir Th Pb Bi Po At Pt Au H g Name given by Mendeleev: 1* Eka - Aluminium, 2* Eka - Boron, 3* Eka - Silicon, 4* Eka - Manganese Importance of Mendeleev’s periodic table 1. It was the first systematic table of classification of elements. 2. It led to the discovery of new elements. Defects of Mendeleev’s periodic table 1. Hydrogen is a non-metal, but it is kept along with metals in the group I. It was not reasonable. 2. When the elements are arranged according to their atomic weight then some dissimilar elements like sodium and gold lie in the same group. 3. He put argon (atomic weight 39.9) before potassium (atomic weight 39.1). It is against the Mendeleev’s periodic law. Modern periodic table Mendeleev’s periodic table had several drawbacks. So, it was corrected by Henery Mosely in 1913. After a number of experiments, he led to a conclusion that the atomic weight is not the fundamental property of an atom. But the atomic number is the fundamental property of an atom. So, he offered the modern periodic law and constructed a scientific table, which is known as a modern periodic table. The modern periodic law states that, the physical and chemical properties of elements are the periodic function of their atomic number. Matter 145

The modern periodic table removed all the drawbacks of Mendeleev’s periodic table. Noble gases Other nonmetals Poor metals Transition metals Alkaline earth metals Alkali metals 146 New Creative Science and Environment Book - 8 Periodic Table of Elements 1 2 3 4 5 6 7 89 10 11 12 13 14 15 16 1 1 1 Atomic # C Solid Metals N onm etals 2 2K 1H Symbol He Hydrogen 1.0079 4 Name Helium Atomic Weight 4 .002602 3 42 2 H gLiquid Lanthanoids 5 62 72 82 92 2 10 2K 2 H G as Actinoids 7 8L 2L i 1 R fU nknown B 3 4 5 6 Lithium 6.9 4 1 Be Boron C N O F Ne 10.8 11 Beryllium Carbon Nitrogen Oxygen Fluorine Neon 9 .01218 2 12.0107 14 .0067 15 .9 9 9 4 18 .9 9 8 4 03 20.179 7 11 2 12 2 13 2 14 2 15 2 16 2 17 2 18 2K 8 8 8 8 8 8 8 8L 4 S 8M 3N a 1 M g2 A l 3S i P5 6C l 7A r Sulfur Sodium Magnesium Aluminium Silicon Phosphorus 3 2.065 Chlorine Argon 3 0.9 73 762 3 5 .4 5 3 3 9 .9 4 8 22.9 8 9 769 2824 .3 05 0 26.9 8 15 3 8 628 .08 5 5 19 2 20 2 21 2 22 2 23 2 24 2 25 2 26 2 27 2 28 2 29 2 30 2 31 2 32 2 33 2 34 2 35 2 36 2K 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8L 4K 8 Ca 1 S8 c T9 i V10 C r M11 13 n F e C o N i C u Z n G a G e A s S e B r K r13 14 15 16 18 18 18 18 18 18 18 18 M Calcium 2 21 2 2 2 2 1 2 3 4 5 6 7 8N 4 0.078 2 2 Potassium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Z inc G allium G ermanium Arsenic Selenium Bromine Krypton 3 9 .09 8 3 5 0.9 4 15 5 1.9 9 61 63 .5 4 6 65 .3 8 69 .723 72.63 74 .9 2160 78 .9 6 79 .9 04 8 3 .79 8 4 4 .9 5 5 9 124 7.8 67 5 4 .9 3 8 04 55 5 .8 4 5 5 8 .9 3 3 19 55 8 .69 3 4 37 2 38 2 39 2 40 2 41 2 42 2 43 2 44 2 45 2 46 2 47 2 48 2 49 2 50 2 51 2 52 2 53 2 54 2K 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8L Sr 5 R b 18 Y18 Z r N b M o T c R u R h Pd18 18 18 18 18 18 18 A g C d I n18 18 18 S n S b T e18 18 18 I18 X e18 18 M 8 Strontium 8 9 10 12 13 14 15 16 1 87.62 2 2 211111 18 18 18 18 18 18 18 18 18 N 012 345 6 7 8O Rubidium Y ttrium Z irconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine X enon 8 5 .4 678 8 8 .9 05 8 5 9 1.224 9 2.9 063 8 95 .96 (9 7.9720) 101.07 102.9 05 5 0 106.4 2 107.8 68 2 112.4 11 114 .8 18 118 .710 121.760 127.60 126.9 04 4 7 13 1.29 3 55 2 56 2 72 2 73 2 74 2 75 2 76 2 77 2 78 2 79 2 80 2 81 2 82 2 83 2 84 2 85 2 86 2K 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8L 5 7– 71 H f 6 C s B a18 18 T a W18 18 R e O s I r18 18 18 Pt18 A u H g T l Pb18 18 18 18 B i18 Po18 A t18 R n18 18 M 18 18 Hafnium 88 178 .4 9 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 N 10 11 12 13 14 15 17 18 18 18 18 18 18 18 18 O Caesium 1 Barium 2 2 Tantalum 2 Tungsten 2 Rhenium 2 Osmium 2 Iridium 2 Platinum 1 G old 1 Mercury 2 Thallium 3 Lead 4 Bismuth 5 Polonium 6 Astatine 7 Radon 8P 13 2.9 05 4 5 1913 7.3 27 18 0.9 4 78 8 18 3 .8 4 18 6.207 19 0.23 19 2.217 19 5 .08 4 19 6.9 665 69 200.5 9 204 .3 8 3 3 207.2 208 .9 8 04 0 (208 .9 8 24 ) (209 .9 8 71) (222.0176) 87 2 88 2 104 2 105 2 106 2 107 2 108 2 109 2 110 2 111 2 112 2 113 2 114 2 115 2 116 2 117 118 2K 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8L 7F r R a18 18 8 9 – 10R3 f Db18 S g B h H s M t Ds18 18 18 18 18 R g C n U ut F l18 18 18 18 U up L v U us18 18 18 U uo18 M Francium 32 N (223 ) 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 O 18 18 32 32 32 32 32 32 32 32 32 32 32 32 32 8 Radium 8 Rutherfordium10 Dubnium 11 Seaborgium 12 Bohrium 13 Hassium 14 Meitnerium 15 Darmstadtium17 Roentgenium 18 Copernicium 18 U nuntrium 18 Flerovium 18 U nunpeniutm 18 Livermorium 18 U nunsepiutm U nunoctium18 P 1 (226) 2 2 (262) 2 (266) 2 (264 ) 2 (277) 2 (268 ) 2 (271) 1 (272) 1 (28 5) 2 (28 4) 3 (28 9) 4 (28 8) 5 (29 2) 6 8Q (261) (29 4) . 57 2 58 2 59 2 60 2 61 2 62 2 63 2 64 2 65 2 66 2 67 2 68 2 69 2 70 2 71 2 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 18 Eu 18 Er L a C e18 19 Pr 18 N d 18 Pm 18 S m 18 25 G d 18 T b Dy18 18 H o 18 T m Y b L u18 18 18 18 18 9 21 22 23 24 Europium 8 25 27 28 29 Erbium 9 2 8 8 8 8 15 1.9 64 2 9 8 8 167.25 9 30 31 32 32 8 8889 Lanthanum 2 Cerium Praseodymium 2 Neodymium 2 Promethium 2 Samarium 2 G adolinium 2 Terbium 2 Dysprosium 2 Holmium 2 2 Thulium 2 Y tterbium 2 Lutetium 2 13 8 .9 05 4 7 14 0.116 14 0.9 0765 14 4 .24 2 (14 5 ) 15 0.3 6 15 7.25 15 8 .9 25 3 5 162.5 00 164 .9 3 03 2 168 .9 3 4 21 173 .05 4 174 .9 668 89 2 90 2 91 2 92 2 93 2 94 2 95 2 96 2 97 2 98 2 99 2 100 2 101 2 102 2 103 2 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 A c T h Pa18 18 U18 N p Pu18 18 A m C m B k C f E s F m M d N o L r18 18 18 18 18 18 18 18 18 18 32 32 18 18 32 32 32 32 32 32 32 32 32 32 32 32 32 20 21 22 24 25 25 27 28 29 3 0 3 1 3 2 3 2 Actinium 9 Thorium 10 Protactinium 9 U ranium 9 Neptunium 9 Plutonium 8 Americium 8 Curium 9 Berkelium 8 Californium 8 Einsteinium 8 Fermium 8 Mendelevium 8 Nobelium 8 Lawrencium 9 2 23 2.03 8 062 23 1.03 5 8 28 23 8 .028 9 21 (23 7) 2 (24 4 ) 2 (24 3) 2 (24 7) 2 (24 7) 2 (25 1) 2 (25 2) 2 (25 7) 2 (25 8) 2 (25 9) 2 (262) 2 (227)