grade-8-science-text-book-61fb9947be91f.pdf

SCIENCE Part - I Grade 8 Educational Publications Department i First Print 2016 Second Print 2017 Third Print 2018 Fourth Print 2019 All Rights Reserved ISBN 978-955-25-0132-6 Published by Educational Publications Department Printed by Neo Graphics (Pvt) Ltd No. 44, Udahamulla Station Road, Gangodawila, Nugegoda. ii The National Anthem of Sri Lanka Sri Lanka Matha Apa Sri Lanka Namo Namo Namo Namo Matha Sundara siri barinee, surendi athi sobamana Lanka Dhanya dhanaya neka mal palaturu piri jaya bhoomiya ramya Apa hata sepa siri setha sadana jeewanaye matha Piliganu mena apa bhakthi pooja Namo Namo Matha Apa Sri Lanka Namo Namo Namo Namo Matha Oba we apa vidya Obamaya apa sathya Oba we apa shakthi Apa hada thula bhakthi Oba apa aloke Apage anuprane Oba apa jeevana we Apa mukthiya oba we Nava jeevana demine, nithina apa pubudukaran matha Gnana veerya vadawamina regena yanu mana jaya bhoomi kara Eka mavakage daru kela bevina Yamu yamu vee nopama Prema vada sema bheda durerada Namo, Namo Matha Apa Sri Lanka Namo Namo Namo Namo Matha iii wms fjuq tl ujlf. orefjda tl ksjfiys fjfikd tl mdge;s tl reêrh fõ wm lh ;=< ÿjkd tneúks wms fjuq fidhqre fidhqßfhda tl f,i tys jefvkd Ôj;a jk wm fuu ksjfia fid¢k isáh hq;= fõ ieug u fu;a lreKd.=fKkS fjdaId - Cøμa\À Musical therapy - ix.S; Ñls;aidj - Cø\a ]Qaø\ Science | Sound 77 6 Magnets There are instances where magnets are used in our day-to-day life. Recalling what we have learnt about magnets in grade 6, let us do Activity 6.1 to identify materials that show magnetic properties. Activity 6.1 You will need:- A permanent magnet, a piece of thread, a stand, various types of coins, an iron nail, a brass nail, a pebble, a plastic ruler, several other things that you like to test for magnetic properties. Method:- ² Hang the magnet on the stand using the piece of thread as shown in Figure 6.1 ² Bring each substance, one at a time, close to the Figure 6.1 magnet, when the magnet remains still. Enter the observations in Table 6.1 Table 6.1 Material Attract / does not attract to the magnet 1. Plastic ruler Does not attract. It will be clear to you that only certain materials attract towards magnets. Materials which attract towards magnets are known as magnetic materials. Metals such as iron, nickel, chromium and alloys like steel and ferrite are magnetic materials. Alloy ferrite is used to make more powerful magnets. Magnets made of Steel Magnets made of Ferrite Figure 6.2 Magnets made of various materials 78 Science | Magnets Magnetic property or magnetism is a property of some materials. 6.1 Poles of a magnet Let us do Activity 6.2 to study further how magnetic power exists around a magnet. Activity 6.2 You will need :- A bar magnet, iron filings, a thin polythene sheet or a polythene bag, a sheet of paper Method:- ² Cover the bar magnet completely with the polythene bag. ² Heap iron filings on the sheet of paper. ² Dip the magnet on the heap of iron filings. ² Take the magnet out of the heap of iron filings and observe the pattern of iron filings attracted to the magnet. Regions where iron filings are thickly attracted can be easily identified. Magnetic power is concentrated in these regions. Figure 6.3 Regions of a magnet, where magnetic power is concentrated are called magnetic poles. There are two of them. ² North pole (N) ² South pole (S) S N S N N S N S Figure 6.4 How poles are located in different types of magnets Identifying magnetic poles We have learnt earlier that north and south poles are marked in most of the magnets. Now, let us consider how the poles of a magnet can be identified when they are not marked. Science | Magnets 79 Figure 6.5 Magnets with poles marked Figure 6.6 A magnet on which poles are not marked Let us do Activity 6.3 to study the methods of identifying the poles of a magnet. Activity 6.3 You will need : - A magnet on which poles are not marked, a magnet on which poles are marked, a compass, a piece of thread, a stand, a piece of cork or a piece of styrofoam, a basin of water, two watch glasses Method : - ² Let us find out various methods to identify the poles of a magnet using given materials. Following methods can be tried out for this. North South Figure 6.7 Using a Figure 6.8 Considering the Figure 6.9 Considering the compass direction that a magnet turns, direction, that magnet turns, when it is hung by a thread. when it is floated on water using a piece of cork or styrofoam. Figure 6.10 Observing the Figure 6.11 Observing the direction attraction or repulsion when a that the magnet turns when it is kept on a magnet with known poles is brought watch glass and moved freely on another closer watch glass Investigate whether there are methods, other than those mentioned, to identify the poles of a magnet. 80 Science | Magnets 6.2 Magnetic field of a magnet Let us do Activity 6.4 to find out about the area that magnetic power is distributed around a magnet. Activity 6.4 You will need : - A bar magnet, iron filings, a piece of cardboard Method : - ² Spread a thin layer of iron filings on the sheet of cardboard. ² Gently place the sheet of cardboard on the bar magnet. ² Tap on the sheet of cardboard gently. ² Observe the pattern in which iron filings are arranged. ² Can you suggest the reason for the arrangement of iron filings on the sheet of cardboard, according to a pattern? Let us do Activity 6.5 to study the magnetic field around a bar magnet. Activity 6.5 You will need : - A bar magnet, iron filings, Glycerine A test tube of the size to insert the magnet, a mixed with beaker of tall form, glycerine or coconut oil iron filings Method : - Bar magnet (inserted ² Fill the beaker with glycerine or coconut oil in a test tube and dipped in the mixed with iron filings. beaker) ² Insert the bar magnet into the test tube and dip it slowly in the beaker. Figure 6.12 A bar magnet ² Observe the pattern of iron filings arranged dipped in glycerine mixed with iron filings around the magnet. It can be observed that iron filings are arranged in a pattern, within a certain area around the magnet. Area that the magnetic power is spread around a magnet is called the magnetic field of that magnet. Imaginary lines used to denote the magnetic power around a magnet are known as magnetic field lines. Figure 6.13 How iron filings are arranged around a bar magnet Let us do Activity 6.6 to demonstrate the magnetic fields between magnetic poles. Science | Magnets 81 Activity 6.6 You will need : - Two short bar magnets, a styrofoam board of A4 size, 4 pieces of cardboard of A4 size, binder gum, iron filings Method : - ² Carve two grooves in the styrofoam board. ² Insert two short bar magnets into the grooves, so that like poles are directed against each other, as shown in Figure 6.14. Figure 6.14 ² Place one piece of cardboard on the styrofoam board. ² Spread a thin layer of iron filings on the cardboard. ² Tap gently to a corner of the cardboard sheet. ² Observe the pattern in which iron filings are arranged. ² Apply a layer of binder gum on another cardboard sheet and allow it to dry. ² Place the side of the cardboard applied with gum, on the pattern of iron filings and press gently. ² Take away the cardboard sheet applied with gum and observe. The pattern of magnetic field lines are imprinted on it. ² Now change the poles of one magnet so that the set-up is changed to demonstrate the magnetic field between unlike magnetic poles. (Figure 6.15) Figure 6.15 ² Repeat the above steps and obtain the pattern of iron filings corresponding to the magnetic field between two unlike poles. ² Exhibit your creations in the classroom. It may be clear to you that iron filings are arranged around a magnet along the patterns of magnetic field lines. Pattern of magnetic field between unlike Pattern of magnetic field between like poles. poles. Figure 6.16 Pattern of magnetic field lines between magnetic poles 82 Science | Magnets 6.3 Compass You may have heard that an instrument called compass is used to find the direction. Compass was invented by Chinese about thousand years ago. Today various types of compasses are in use. A compass is made from a magnetic needle (this is like a small magnet) which can freely float on a liquid or turn round on a pivoted point. Figure 6.17 Types of compasses Let us do Activity 6.7 to make a simple compass. Activity 6.7 You will need : - A large needle, a cork bung, a small knife, a bar magnet, a plastic basin full of water, red paint. Method : - ² Magnetize the needle by contact method using the bar magnet. ² Cut a thin slice of the cork bung and fix the needle on it. (Figure 6.17) ² Float the slice of cork, with the needle on the basin of water. ² Test whether the floating needle is always turned in the same direction. ² Colour the end of the needle, which always turns to the geographical north with red paint. ² What you have constructed is a simple compass. ² Modify your compass to make it more attractive. Figure 6.18 Making a compass out of a needle. Science | Magnets 83 Figure 6.19 Several compasses constructed in various ways When a compass is kept near a magnet, the needle turns along the direction of the magnetic field. Therefore, the magnetic field of a magnet can be identified, using a compass. Let us do Activity 6.8 to identify the direction of magnatic field using a compass. Activity 6.8 You will need : - A bar magnet, a compass, a sheet of white paper Method : - ² Place the bar magnet on the sheet of white paper. ² Draw the outline of the magnet on the paper, using a pencil. ² Label the north and south poles of the magnet on the paper. ² Place the compass on the paper as shown in Figure 6.20 and mark the positions of the compass needle. ² If you are unable to find several compasses, you can use the same compass for each location. ² Try to build up the pattern of the magnetic field by connecting the positions of the compass needle. Figure 6.20 Positions of a compass needle around a bar magnet at various locations 84 Science | Magnets Magnetic field lines of a permanent magnet direct from North pole to South pole. Hence, the direction of magnetic field is from North pole to South pole. The Figure 6.21 illustrate the arrangement of magnetic field lines around a bar magnet. N S Figure 6.21 Magnetic field of a bar magnet 6.4 Geomagnetism You know that north - south directions of the earth can be identified using a compass. When a compass is kept horizontally near the surface of the earth, its needle turns along the north-south direction. Let us do Activity 6.9 to find the direction of the magnetic field of the earth. Activity 6.9 You will need : - Two compasses, a bar magnet, a piece of thread, a stand Method : - ² Hang one bar magnet horizontally on the stand, using the piece of thread. ² Keep the bar magnet, hung on the stand, and two compasses about two meters apart from each other. ² Take another bar magnet and bring one of its poles closer to each compass and to the magnet hung on the stand. ² Record your observations. ² Take away the bar magnet and observe the directions of the poles of compass needles and the bar magnet which is hung. ² Repeat the activity, changing the locations of compasses. ² Discuss the reasons for the observations in the classroom. Figure 6.22 Science | Magnets 85 The compasses and the magnet which is hung turned when another magnet is brought closer to them. Thus it is clear that magnets and compasses turn, when they are under the influence of a magnetic field. When bar magnets and compasses are free from the influence of other magnets, their north poles always turn to one direction and south poles to the opposite direction. Though the position of bar magnets and compasses are changed, their poles turn to the same directions. The reason for this is the existence of a large magnetic field around the earth through north and south poles. This magnetic field existing near the earth is known as geomagnetism. Liquified metal currents circulate around the axis of the earth, because True North of the high temperature at the core Magnetic North of the earth. The magnetic field of Field lines the earth is the result of the electric S currents thus generated. When a compass or a magnet is kept N freely near the earth, its north and south poles are directed along the Magnetic magnetic field of the earth. True South South The direction that the north pole of Figure 6.23 How earth's magnetic field is a magnet or a compass, kept in that located manner is known as the magnetic Figure 6.23 How earth's magnetic north of the earth. field is located magnetic North True North There is a little difference between the real north and the magnetic north of the earth. The magnetic north lies a few degrees north west from the real north. Figure 6.24 How the magnetic North and real North are denoted on a map 6.5 Temporary magnets and permanent magnets Two types of magnets can be identified when considering the uses of magnets. ² Permanent magnets ² Temporary magnets Let us do Activity 6.10 to understand more about permanent magnents and temporary magnets. 86 Science | Magnets Activity 6.10 You will need : - Iron nail or iron rod of about 2 inch length, two meters of insulated copper wire of 32 SWG, two dry cells, cellotape, a bar magnet, few file clips or pins, a switch Method : - ² Wind the insulated copper wire of 32 SWG, around the iron nail or iron rod, to make a coil. ² Scrape both ends of the coil and connect it to the dry cells. ² Bring the coil close to the file clips while supplying electrcity and see what happens. ² Disconnect the electrical supply and bring the coil close to the clips, again ² Bring the bar magnet close to the clips and see what happens. ² Discuss your observations in the classroom. Figure 6.26 File clips Figure 6.25 File clips are Figure 6.27 File fall off (do not attract) attracted when electricity clips are attracted to a when electrical supply is is supplied permanent magnet disconnected A set-up that becomes a magnet, only when electricity is supplied is known as an electromagnet. In an electromagnet, magnetic power remains only when electricity is supplied. Therefore, they are called temporary magnets. Magnetic power remains for a long time in bar magnets. Therefore, they are called permanent magnets. Making a permanent magnet Magnets of various shapes and sizes are used for various purposes. Let us consider how these magnets are constructed. Materials that show magnetic properties are used to make magnets. Steel, ferrite and soft iron are some magnetic materials which are used to make magnets. Various materials are used to produce various types of magnets. Magnetic power is not retained in soft iron for a long time. Therefore, soft iron is used to make electromagnets and other temporary magnets. Science | Magnets 87 Magnets, in which magnetic power is retained for a long time, are known as permanent magnets. Steel or ferrite is used to make permanent magnets. Ferrite is used to make more powerful permanent magnets. Figure 6.28 An electromagnet Figure 6.29 Permanent Figure 6.30 Permanent magnets made of steel magnets made of ferrite Construction of permanent magnets using magnetic materials can be done in two ways. 1. Electrical method 2. Contact method Let us do Activities 6.11 and 6.12 to make magnets using electrical method and contact method. Activity 6.11 You will need :- Iron nail or iron hacksaw blade of 2 inches, two meters of insulated copper wire of 32 SWG (Standard Wire Gauge), two dry cells, cellotape, a piece of cardboard, few file clips Method : - ² Make a 5 cm long tube (about the size of a pencil) using the piece of cardboard. Figure 6.31 ² Wind the copper wire of 32 SWG, around that tube to make a coil. ² Bring the iron nail close to the file clips to check whether it has magnetic power. ² Then insert the iron nail into the cardboard tube. ² Scrape both ends of the coil and connect it to the dry cell and supply the current several times to the circuit. ² Take the iron nail/ iron hacksaw blade away and observe while bringing it close to the file clips. ² Discuss your observations in the classroom. 88 Science | Magnets The electric current should be supplied several times to the circuit for a long time until permanent magnetism is observed. Activity 6.12 You will need : - A steel needle or steel hacksaw blade of two inches, a few file clips, a bar magnet Method : - ² Bring the needle/hacksaw blade close to the file clips to check whether it has magnetic power. ² Now, place the needle horizontally on a table. ² Place one end of the bar magnet on Figure 6.32 the needle and drag it along the same direction as shown in Figure 6.32. ² Repeat this process several times. ² Now bring the needle/hacksaw blade close to the file clips and see what happens. ² Discuss your observations in the classroom. It will be clear to you that a permanent magnet can be made using electrical method and contact method according to Activities 6.11 and 6.12. Does the magnetic power of permanent magnets retain forever ? The answer is 'No'. The magnetic power of permanent magnets are lost due to various reasons. Some of the reasons are given below. ² Ageing ² Being subjected to high temperatures ² Being subjected to strong magnetic fields ² Being subjected to vibrations Let us do Activity 6.13 to test how magnetic power is lost. Rubbing should be continued for a longtime until permanent magnetism is observed. Science | Magnets 89 Activity 6.13 You will need : - Three identical iron nails magnetized by a permanent magnet, a few pins, a bunsen burner, a hammer, a pair of crucible tongs, a strong permanent magnet Method : - ² Bring the pins close to each magnetized iron nail, separately, and note down the maximum number of pins attracted to each nail. ² Subject each nail to each of the following treatments. (a) Vibrate by hammering. (b) Heat to a high temperature. (c) Move to and from close to the strong magnet. ² Bring the pins close to each nail again and count the number of pins attracted to each nail. Fill Table 6.2. Figure 6.33 Heating Figure 6.34 Subjected to Figure 6.35 Subjected to strongly vibrations strong magnetic fields Table 6.2 Action done Number of pins attracted Number of pins attracted before action after action Hammering Heating Subjecting to strong magnetic fields It may be clear to you that magnetic power fades off because of vibrations, temperature and being subjected to strong magnetic fields. Magnetic power also fades due to ageing. Magnets should be stored in an orderly manner without being subjected to vibrations, temperature and strong magnetic fields to maintain magnetic power for a long time. 90 Science | Magnets Storage of permanent magnets Magnetic power of a permanent magnet can be protected for a long time, if it is stored in such a way that its magnetic field does not scatter. Soft iron plate Strap of wood Soft iron plate Soft iron plate Bar magnets Figure 6.36 How magnets are stored Use of permanent magnets There are various equipments found in day-to-day life, where permanent magnets are used. Assignment 6.1 List out instances where permanent magnets are used. Check whether permanent magnets are used in the following instances. In loud speakers and speakers In small electric motors In some door locks Bags In some toys In compasses Science | Magnets 91 In pencil boxes Stickers on refrigerators Phone covers Figure 6.37 Some applications of permanent magnets For extra knowledge There are permanent magnets as well as electro-magnets found in most of the small electric motors. But there are some motors only with electro-magnets. Permanent magnets Electro magnets Summary ² Magnetism is a property of some materials. ² Materials which attract to magnets are magnetic materials. ² Iron, nickel, chromium, steel and ferrite are some examples for magnetic materials. ² The area that the magnetic force exists around a magnet is called the magnetic field. ² Imaginary lines used to denote the influence of magnetic field are known as magnetic field lines. ² The direction of magnetic field is from the north pole to the south pole. ² A compass is important to detect magnetic fields. 92 Science | Magnets ² Terminals of a magnet, where magnetic force is concentrated are called magnetic poles. ² There is a magnetic field on the earth. It is known as geomagnetism. When a compass is placed near the earth the direction that its pointer indicates is the direction of earth's magnetic field. ² The direction indicated by the compass is the magnetic north. It lies a little north-western to the real north. ² Permanent magnets are made of steel and ferrite, and temporary magnets are made of soft iron. ² Contact method and electrical method are used to make permanent magnets. ² Power of a magnet may wear off with time, because of high temperature, strong vibrations and the influence of strong magnetic fields. ² Power of a magnet can be retained for a long time by proper storage. ² Permanent magnets and electromagnets are widely used in day-to-day life. Exercise 1. Select the appropriate words from the brackets and fill in the blanks of the paragraph given below. (Soft iron, magnetic materials, magnetic poles, magnetic field lines, ferrite, magnetic field). Materials that show magnetic properties are called.............................. The best material to make permanent magnets is............................... To make temporary magnets,........................................... is commonly used. The area in which magnetic forces exist is called.............................. Influence of a magnetic field can be observed using........................................ The area on a magnet, where the magnetic forces are concentrated is known as the.......................... 2. Given below is a rough sketch of a pencil box that closes with the help of a magnet. Suggest a method to test whether the magnet is fixed on the box or on the lid. 3. A student who checked some magnets in the school laboratory found out that their magnetic force is worn out. Give three reasons for that. Science | Magnets 93 4. Explain scientific reasons for the following. (a) North pole of a bar magnet, hung freely by a thread is directed towards north. (b) A piece of iron is attracted towards a magnet, but a piece of copper is not. 5. An iron rod, placed on a table was contacted several times with a bar magnet. Then, it was observed that pins and small pieces of wire are attracted to the iron rod. (a) Give reasons for the above incident. (b) What is the term used for the above process? (c) Suggest another method to get the same result without using a permanent magnet. Technical Terms Magnet - pqïnl - Põ¢u® Permanent magnet - iaÓr pqïnl - {ø»¯õÚ Põ¢u® Magnetic field - pqïnl lafIa;%h - Põ¢u¨¦»® Geomagnetism - N+ pqïnl;ajh - ¦ÂUPõ¢uÂ¯À Compass - ud,sudj - vø\Põmi Electromagnet - úoHq;a pqïnl - ªßPõ¢u® Magnetic pole - pqïnl O%ej - Põ¢u øÚÄ Magnetic materials - pqïnl øjH - Põ¢uzvμÂ¯® Steel - jdfka - E¸US Ferrite - f*rhsÜ - ö£øμØÖ Soft iron - uDÿ hlv - ö©ßÛ¸®¦ North pole - W;a;r O%ejh - Áh øÚÄ South pole - olaIsK O%ejh - öuß øÚÄ 94 Science | Magnets 7 Measurements Associated with Electricity Electricity is one of the main sources of energy used in day-to-day life. Recalling what we have studied about electricity in lower grades let us do Activity 7.1. Activity 7.1 You will need:- Two dry cells, a torch bulb, a switch, a bulb holder, connecting wires Method:- ² Prepare a circuit to light the torch bulb using the given items. ² Switch on your set-up and observe what happens. ² Draw the set-up you prepared using circuit symbols. ² Mention the positive and negative terminals of the cells correctly on Figure 7.1 the diagram you draw. ² Discuss the reason for the illumination of bulb. The electric current produced in the cells when the switch is closed flows through the conductors of the circuit. The bulb is illuminated because current flows through it. Flow of electrical charges through a closed circuit is known as an electric current. 7.1 Electric current Let us do Activity 7.2 to study the flow of electric current through a conductor. Science | Measurements Associated with Electricity 95 Activity 7.2 You will need:- Two dry cells, a switch, a small motor, connecting wires Method:- ² Prepare the circuit as shown in Figure 7.2 ² Connect the parts as indicated in Table 7.1 and switch on the circuit. ² Record your observations. Figure 7.2 Table 7.1 Table 7.1 Observations Step Observations after changing the terminals of cells 1. Connect the electric motor It rotates to one direction................................. ² What happens when terminals of cells are changed ? ² What can be concluded according to your observations ? The direction of the current flow, changes when the terminals of the cells are changed. The reason for the change of rotational direction of the motor is the change of the direction of current. ² There is a definite direction for the flow of electric current. ² Conventionally, it is considered that current flows from the positive terminal to the negative terminal. A center-zero galvanometer or a center-zero ammeter/ milliammeter can be used to identify the direction of an electric current. Figure 7.3 A galvanometer Figure 7.4 A milliammeter 96 Science | Measurements Associated with Electricity Let us do Activity 7.3 to study further about the direction of current. Activity 7.3 You will need:- An ammeter or center-zero milliammeter, an electric motor, a dry cell, a switch Method:- ² Prepare the circuit as in Figure mA 7.5. ² Operate the circuit and observe what happens. ² Interchange the terminals of the cell and observe again. Figure 7.5 ² Draw diagrams for each instance and mark the direction of the current flow. ² Discuss the reason for your observations. It is clear that, when changing the connecting terminals to the battery, the direction of motion of ammeter-indicator and the rotational direction of the motor are changed. The reason for this is the change of the direction of current. Measuring the electric current Physical quantities are measured in various instances. For this purpose various measuring equipment and various units are used. Electric current is also a physical quantity. Let us investigate how electric current is measured. Symbol for electric current - I International unit (SI) for electric current - Ampere Symbol - A Sub units are used to measure small currents. Two such sub units are given below. ² Milliampere - mA ² Microampere - μA 1000 mA - 1A 1000 μA - 1 mA Equipment used to measure current - Ammeter + - Symbol - A Milliammeter or microammeter can be used to measure small electric currents. Science | Measurements Associated with Electricity 97 There are two terminals, positive and negative, in ammeter and milliammeter. Usually the positive terminal is red and negative terminal is black. ² When an ammeter is used in a circuit the terminals should be connected correctly. ² To measure the current, ammeter or milliammeter is connected in series to the circuit. Figure 7.6 Ammeter Figure 7.7 Milliammeter Let us do Activity 7.4 to measure the current flowing through a circuit. Activity 7.4 You will need:- Two dry cells, six torch bulbs, bulb holders, connecting wires, switches, an ammeter, a milliammeter Method:- ² Prepare the circuit as in mA Figure 7.8. ² Connect the milliammeter to the circuit. A ² Measure the current flowing through the bulb while it is illuminating. ² Draw the circuit, to which the milliammeter is connected, using symbols. Figure 7.8 ² Connect the ammeter instead of the milliammeter and take the readings again. ² Connection of which instrument makes it easier to take the readings ? ² Is it ammeter or milliammeter ? ² Discuss the reason for your answer in the classroom. 98 Science | Measurements Associated with Electricity The electric current flow through the above circuit is lesser than one amphere (1A). Therefore, it is suitable to use miliammeter to measure small currents. Ammeter is suitable to measure large currents, while milliammeter is suitable to measure small currents. Let us consider another factor, essential for flowing of electric current through a conductor. 7.2 Potential difference Waterfall Pond water Dry cell Bulb Water pump Figure 7.9 You may have seen ponds and waterfalls designed in modern houses, which function with the help of water pump. Pond water has less potential energy. But when water is pumped up to the waterfall more potential energy is stored. The process of electric circuit takes place in the same manner. Dry cell provides electric potential energy to electric chargers. Positive (+) terminal has higher potential than the negative (-) terminal. This difference of electric potential energy between the two terminals of the cell is called voltage or potential difference. Electic current flows from a higher electric potential to a lower electric potential. The voltage between positive terminal and negative terminal of electric cells and batteries is marked on them. Science | Measurements Associated with Electricity 99 Assignment 7.1 ² Collect as many as possible electric cells and batteries used commonly. ² Observe how the positive and negative terminals and the voltage values are marked on them. ² Prepare a table of the cells you collected and their voltages. Figure 7.10 How voltage is marked on some cells Measuring the potential difference Symbol for potential difference - V International unit (SI) for potential difference - Volt Symbol - V Equipment used to measure potential difference - Voltmeter + - Symbol of voltmeter - V There are positive and negative terminals in voltmeter as well as in ammeter. Usually, positive terminal is red and negative terminal is black. Voltmeter is connected parallely to the circuit to measure the potential difference between two points. Figure 7.11 Voltmeter Let us do Activity 7.5 to identify the voltages of some cells and batteries which are commonly used. 100 Science | Measurements Associated with Electricity Activity 7.5 You will need:- Several dry cells, a button cell, a voltmeter, connecting wires Method:- ² Observe how the voltages are marked on the cells and batteries you collected. ² Connect the cells or batteries to the circuit you made as shown in Figure 7.12 ² Measure the voltage between the terminals of the cells V or batteries using the voltmeter. ² Compare the values obtained by measuring and the values mentioned. Figure 7.12 ² Tabulate your observations. Table 7.2 Cell/ Battery Voltage (V) Dry cell Lead acid accumulator Button cell The voltage of a normal dry cell is 1.5 V. The voltage between the terminals of a car battery containing six cells is 12 V. Let us do Activity 7.6 to measure the potential difference between two points of a circuit, using a voltmeter. Activity 7.6 You will need:- Two dry cells, a torch bulb, a bulb holder, a small electrical motor, a voltmeter, connecting wires, a switch V V Figure 7.13 Figure 7.14 Science | Measurements Associated with Electricity 101 Method:- (A) ² Build a circuit to light the bulb, using the bulb, two dry cells, and a switch. ² Connect the voltmeter correctly to measure the potential difference between the two ends of the bulb. ² Measure and record the potential difference between two ends of the bulb ² Draw the diagram of the circuit you built using symbols. (B) ² Remove the bulb and connect the electrical motor to the circuit. ² Switch on the circuit and measure the potential difference between the terminals of the motor. (C) ² Connect both, the bulb and the motor to the circuit as shown in Figure 7.14 ² Measure separately the potential difference between the terminals of the bulb and the motor, using the voltmeter. Now you have the ability of measuring the potential difference between two points of a given electrical circuit. There are instances, in day-to-day life where accurate measurements of current and voltage have to be taken. Some such instances are given below. 1. To make sure, voltages, supplied to houses and factories are of the accurate voltage. 2. To detect defects of electrical appliances by measuring the current they consume. 3. To take measurements associated with electricity in power houses and electrical generators. 4. To identify whether the parts of electrical appliances are functioning properly when repairing. Figure 7.15 Repairing electrical Figure 7.16 Measuring electricity appliances in power houses and electrical generators 102 Science | Measurements Associated with Electricity For extra knowledge Very sensitive voltmeters and ammeters, assembled using digital technology, are in use currently. They are very high in sensitivity. Reading has been given on the board digitally. Therefore it is easy to use. Modern voltmeters and ammeters assembled using digital technology 7.3 Resistance of a conductor We have already observed that a current flows when a potential difference is applied to the ends of a conductor. Let us find out further, whether there are any other factors affecting the flow of current through a conductor. Activity 7.7 You will need:- Two dry cells, an ammeter, a torch bulb, a bulb holder, a switch, three wires of iron, nichrome and copper of the same length (about 50 cm) and same diameter Method:- ² Prepare a circuit as shown in the figure. A B ² Connect each piece of wire, separately to A and B terminals and switch on the circuit. ² Record the observations in Table 7.3 Figure 7.17 ² Discuss the reasons for your observations in the classroom. Table 7.3 Nature of illumination of Ammeter reading Type of wire the bulb (Ampere) 1. Copper Illuminate brightly....................................... 2. Iron.............................................................................. 3. Nichrome.............................................................................. Science | Measurements Associated with Electricity 103 The reason for the difference in illumination of the bulb is because of the current flowing through the circuit changes depending on the type of conductor used. ² Electric current flowing through a conductor depends on the material that it is made of. ² The reason is that the obstacle for flowing of electric current is different from conductor to conductor. The obstacle caused by a conductor to the flowing of current through it is called the resistance of that conductor. Symbol used to denote resistance - R Unit of measuring resistance - Ohm (Ω) When the resistance of a conductor increases the current flowing through it decreases. For your attention ² Resistance is a very useful factor to control the current flowing through a conductor. ² Current flowing through a conductor can be controlled by changing its resistance. ² Components called resistors, produced to various values of resistance are connected to circuits to control the current flow. ² Mostly the value of a conductor is mentioned on it according to a colour code system. Electrical parts that possess the property called resistance are known as resistors. Some of those components are given in Figure 7.18. Figure 7.18 Various types of resistors Symbols for resistors Now you may understand that the current flowing through a circuit can be reduced by connecting resistors to increase resistance of the circuit. 104 Science | Measurements Associated with Electricity Summary ² The flow of electrical charges through a conductor is known as an electric current. ² Unit of measuring current is Ampere. The equipment used for that is ammeter. ² Ammeter should be connected in series to the circuit. The terminals also should be connected correctly. ² There should be a potential difference between two points of a circuits, for the flow of current. ² Potential difference between the two terminals of an electric source is known as its voltage. ² Unit of measuring potential difference is Volt and the equipment used is voltmeter. ² To measure the potential difference across a part of a circuit the voltmeter should be connected in parallel to it. ² Obstruction of electric current flow through a conductor is known as its resistance. ² Unit of measuring resistance is Ohm. ² Resistors of various values can be used to change the current flowing through a circuit. Exercise 1) Complete the following paragraph using suitable terms for the blanks. Electric current is a flow of................................ through a closed circuit. Always electric current flows from a high...................................... to a........................... electric potential................................... terminal is the place of a cell, where electric potential is high and.................................... terminal is the place where it is low. 2) Figure below shows a set-up prepared by a student to measure the potential difference between two ends of a bulb. 1. Is the circuit suitable for the purpose? 2. Give reasons for your answer. 3. If there is any defect, correct it and draw the circuit again. 4. Mention two facts, that should be considered when connecting a voltmeter to a circuit. Science | Measurements Associated with Electricity 105 3) Given below is a circuit prepared by a student to rotate a cardboard disc using an electric motor. To decrease the rotational speed of this motor; 1. What property of the circuit should be increased? 2. Suggest a method to do it. 4) List out three instances in day-to-day life where measuring voltage and current is important. Technical Terms Electric current - úoHq;a Odrdj - ªß÷Úõmh® Electricity - úoHq;h - ªß\õμ® Electric potential - úoHq;a úNjh - ªß AÊzu® Voltage - fjda,aàh;djh - ÷ÁõÀØÓÍÄ Resistance - m%;sfrdaOh - uøh Resistor - m%;sfrdaOl - uøh° Circuit - mßm:h - _ØÖ Conductor - ikakdhlh - Phzv Voltmeter - fjda,aÜógrh - ÷ÁõÀØÖ©õÛ Switch - iaúph - BÎ 106 Science | Measurements Associated with Electricity 8 Changes in Matter 8.1 Physical changes and chemical changes Tear a paper into small pieces. Burn another piece of paper. Figure 8.1 Can you explain the difference between these two changes? Though the paper is torn into pieces, it is still a paper. So, when tearing a paper its composition is not subjected to any change. Something which is not a paper cannot be formed by tearing the paper. Therefore, such changes are known as physical changes. Changes in which the composition of matter does not change, even though its nature of existence changes, are called physical changes. However, when the paper is burnt, ash and smoke are formed. There the composition of the paper changed and new substances are formed. Such changes are known as chemical changes. Changes in which the composition of matter forming new substances are known as chemical changes. Science | Changes in Matter 107 Let us engage in Activity 8.1 to study the nature of physical changes. Activity 8.1 You will need:- A beaker, water, salt, tripod, spirit lamp/ bunsen burner Method:- ² Take 250 ml beaker and add about 50 ml of water into it. ² Add about one teaspoon of powdered salt into it and dissolve thoroughly. ² Keep a wire gauge on a tripod and place the beaker on it. ² Heat the beaker using the spirit lamp/bunsun burner until water is completely vapourised. Figure 8.2 ² Record your observations. A residue can be seen at the bottom of the beaker. That residue is the salt that was previously dissolved in water. From this it is clear that the change happening during the dissolving of salt in water is a physical change. Let us do Activity 8.2 to investigate the nature of chemical changes. Activity 8.2 You will need:- A magnesium ribbon, a candle or a spirit lamp Method:- ² Take a magnesium ribbon and clean it well. ² Burn it by holding to the flame. ² Record your observations. Figure 8.3 Before burning, the magnesium ribbon had a metallic lustre. When held to the flame, it burnt with a bright flame leaving a white powder. Here, the composition of magnesium has changed and a new substance has formed. Therefore, burning of the magnesium ribbon is a chemical change. Like this, the changes we experience in our day-to-day life can be divided into two types, physical changes and chemical changes. Engage in Assignment 8.1 to reinforce your knowledge in this regard. Assignment 8.1 Classify the following changes as physical changes and chemical changes. ² Melting of solid wax ² Vapourisation of water ² Rusting of iron ² Melting of ice ² Breaking granite into pieces ² Burning camphor ² Burning firewood ² Lighting a cracker 108 Science | Changes in Matter 8.2 Changes of state as physical changes Let us do Activity 8.3 to gain an understanding about the changes of state. Activity 8.3 You will need:- A beaker, a tin lid, a bunsen burner, a glass plate, a tripod, a wire gauge, a crucible, a glass funnel, boiling tubes, surgical spirit, water, a piece of wax, naphthalene, iodine Method:- Do the activities as indicated in Table 8.1 and record relevant observations. Table 8.1 Activity Observation 1. Place the piece of wax in a boiling tube and heat. Observe. Allow to cool and observe again. 2. Put some pieces of ice into a beaker and heat. Observe. Continue heating even after the piece of ice completely turns into water. Make your observations. Hold the plate of glass over the beaker when water boils. (Do as a teacher demonstration) 3. Put a few pieces of iodine into a crucible and heat. Hold an inverted funnel a little above the crucible. You would have observed that the wax melted when it was heated in a boiling tube. You would have also observed that liquid wax turns into solid when it is allowed to cool. When a solid substance is heated, it turns into the liquid state at a certain temperature. The transition of a substance from the solid state to the liquid is called melting or fusion. Transition of a substance from the liquid state to solid state is called freezing. You would have observed that ice turns into water. Ice is a substance that exists in the solid state. Water is a liquid. The conversion of a substance from the solid state to liquid state is also a change of state and it is known as fusion. When that water is heated further water vapourises. The change of a liquid into a gas is known as vapourisation. When water boils, formation of droplets of water on the glass plate can be observed. These droplets were formed by the cooling of steam. The conversion of a substance that exists in the gaseous state into liquid state is called condensation. When crystals of iodine were heated in a crucible, you would have seen that iodine turned directly into a gas. When that iodine gas was brought into contact with a glass surface, crystals of iodine can be seen on the surface from this, it is clear that Science | Changes in Matter 109 when iodine vapour cools it directly turns into solid iodine without becoming a liquid. The turning of a solid into vapour without passing through the liquid state is also a change of state. It is known as sublimation. During a change of state no new substances are formed by changing the composition. Therefore, the changes of state are physical changes. Solid n Fu io s at io Fr im n ee bl zi Su ng Vaporisation Gas Liquid Condensation Figure 8.4 8.3 Chemical changes So far we have discussed about the nature of physical changes. When a physical change occurs, a change in the composition of the substances does not occur. But when chemical changes occur, new substances are formed. Let us do Activity 8.4, 8.5, 8.6 and 8.7 to study the nature of chemical changes further. Activity 8.4 You will need:- Lead nitrate, a boiling tube, a bunsen burner Method:- ² Take about 1g of lead nitrate to a boiling tube. ² Heat the boiling tube using the bunsen burner. ² Record your observations. When white lead nitrate is heated a brown coloured gas is evolved leaving a red coloured powder in the boiling tube. Since, the composition of lead nitrate has changed this is a chemical change. 110 Science | Changes in Matter Activity 8.5 You will need:- Copper sulphate, an iron nail, a boiling tube, a thermometer Method:- ² Add water and copper sulphate crystals to the boiling tube and prepare a light blue solution. ² Put the cleaned iron nail into it. ² Record your observations. When a cleaned iron nail is placed in a copper sulphate solution you would observe that the blue colour of the solution decreases, a reddish brown subtance deposits on the nail and at the bottom while the temperature rises. Activity 8.6 You will need:- A solution of copper sulphate, a solution of sodium hydroxide, two test tubes Method:- ² Mix the copper sulphate solution with the sodium hydroxide solution. ² Record your observations When the copper sulphate solution is added to the sodium hydroxide solution, a formation of a light blue solid can be observed. Such solids are called precipitates. Activity 8.7 You will need:- Dilute hydrochloric acid, a zinc granule, a boiling tube Method:- ² Add a little dilute hydrochloric acid to the boiling tube. ² Add the piece of zinc into it. ² Record your observations. When a granule of zinc is added into hydrochloric acid, we see that zinc dissolves and a gas is liberated. Pay your attention to the above activities. In all of them new substances are formed. You already know that in chemical changes new substances are formed. In the above activities, identify the observations which testify the formation of new substances and complete Table 8.2. Science | Changes in Matter 111 Table 8.2 Reaction Observations in support of the formation of new substances 1. Heating lead nitrate Formation of a red powder Evolution of a brown coloured gas 2. Putting an iron nail into a copper sulphate solution 3. Adding copper sulphate solution to sodium hydroxide solution 4. Adding a zinc granule to hydrochloric acid Based on the observations made with regard to the chemical reactions stated in this chapter before, some of the following can be given as evidences in support of the fact that a chemical reaction has taken place in the above activities. ² Evolution of gases ² Change in colour ² Change in temperature (exchange of heat) ² Formation of precipitates ² Production of sound/light ² Production of an odour The formation of a new substance having a different composition or several new substances by one or more substances undergoing change is known as a chemical change or a chemical reaction. Recall the burning of magnesium again. Magnesium is a metal with a silvery lustre. Upon heating, it combines with oxygen in the air and forms a white powder. That powder is known as magnesium oxide. The substances that get subjected to change during a chemical reaction are called reactants. Hence, the reactants of the above reaction are magnesium and oxygen. The new substances formed by a chemical reactions are referred to as products. The product of this reaction is magnesium oxide. This reaction can be shown in the form of a word equation as follows. Magnesium + Oxygen Magnesium oxide Hence, in a chemical reaction, reactants turn into products. Rusting of iron, tarnishing of metals, combustion of materials, decay of organic matter, ripening of fruits, blast of a cracker and digestion of food by enzymes are some chemical reactions taking place every day. 112 Science | Changes in Matter Law of conservation of mass What kind of a change do you think will happen to the total mass of the substances that are subjected to the chemical changes or chemical reactions you have identified? To inquire into this let us do following activities. Activity 8.8 You will need:- Iron wool, two identical iron wires, a horizontal rod Method:- ² Take two equal masses of iron wool and lump them loosely ² Using the two iron wires tie them to the horizontal Figure 8.5 rod as shown in Figure 8.5. ² Suspend the rod on a support to balance it horizontally. Light one lump of iron wool. ² Record your observations. Iron wool burns giving reddish sparks. At the same time the side with burnt wool moves down. From this we can infer that when iron wool turns into the products of combustion, the mass increases. Activity 8.9 You will need:- A few heads of matches, a boiling tube Method:- ² Put a few heads of matches to a boiling tube. Weigh the boiling tube with them. ² Heat the boiling tube strongly with an open flame until the match heads catch fire. ² After cooling, weigh the boiling tube with its contents. ² Record your observations. Here, you will be able to observe that the mass after the reaction is lower than the mass before the reaction. Here, you may have the problem why there was an increase in the mass when iron wool was burnt in Activity 8.8 while a decrease in mass was shown when the match heads were burnt in Activity 8.9. In the above experiments, the substances were burnt in open environments. Therefore, when those substances react there is a chance to combine with some substances in the environment and also to release the products of combustion to the environment. An increase in mass occured due to addition of some substances. A decrease in mass was noticed due to the loss of some substances to the environment. Science | Changes in Matter 113 Open systems - The systems in which the substances can exchange between the system and the surroundings are referred to as open systems. Closed systems-The systems in which the substances cannot exchange between the system and the environment are called closed systems. Therefore, to find out whether a change occurs in the total mass of substances taking part in a chemical reaction, the experiment should be conducted in a closed system in which substances are neither gained from nor lost to the surrounding. Let us engage in Activity 8.10 and Activity 8.11 which have been designed after taking these facts into consideration. Activity 8.10 You will need:- A few matches, a boiling tube, a rubber balloon Balloon Method:- Boiling ² Let us now conduct Acitvity 8.9 in a closed system. tube ² As shown in Figure 8.6, close the mouth of the boiling Match tube containing matches with a balloon. Measure its sticks mass. Figure 8.6 ² Apply heat close to the bottom of the tube until the matches light up. ² Allow the boiling tube to cool and weigh again. When the matches burn, the balloon get inflated gradually. During the reaction the products are not lost. Also it is seen that there is no change in the total mass before and after the reaction. Activity 8.11 You will need:- A conical flask, lead nitrate 1 g, water 20 ml, sodium chloride 1 g, a boiling tube Method:- ² Take about 1g of lead nitrate to a conical flask and dissolve in about 20 ml of water. ² Take about 1g of sodium chloride to a test tube, Sodium dissolve it in about 5ml of water and transfer chloride this solution to an ignition tube. Lead nitrate ² Tie the ignition tube with sodium chloride Figure 8.7 solution with a string and suspend it inside the conical flask containing the lead nitrate solution with the help of a stopper as shown in Figure 8.7. 114 Science | Changes in Matter ² Seal the conical flask by applying vaseline around the stopper. Weigh the flask with its contents. ² Slant the apparatus slowly and let the two solutions mix. Record your observations. ² Weigh the apparatus again and note the mass. Formation of a white precipitate on mixing the two solutions indicates the occurrence of a chemical reaction in the apparatus. The result of the experiments also shows that there is no change in the total mass before and after the reaction. The french scientist Antoine Lavoisier (1743 - 1794) who conducted many experiments such as the above in relation to various chemical reactions showed for the first time that the total mass of the substances taking part in a chemical reaction (reactants) is equal to the total mass of the products obtained after the reaction. Later this finding came to be known as the Law of conservation of mass. Law of conservation of mass During chemical reactions the total mass does not change. That means the mass is conserved. 8.4 Combustion When magnesium burns in air, magnesium reacts with oxygen in the air forming magnesium oxide. Oxygen gas in air is essential for combustion. Oxygen is the gas in air that supports combustion. There are substances which can be burnt and which cannot be burnt. The substances that can be burnt are known as combustible substances. The substances that cannot be burnt are non-combustible substances. combustible substances: e.g. :- camphor, wax, sulphur, sugur, lacquer, paper, tar, flour, petrol, kerosene non-combustible substances: e.g. :- asbestos, glass, sand, rock Combustion is the reaction of a combustible substance with a gas which acts as a supporter of combustion. The special feature of the reaction of combustion is that it is a chemical change which takes place releasing thermal energy and light energy. A combustible substance has to be heated to a certain temperature for combustion (to start to reacting with oxygen gas). This temperature changes from substance to substance. The temperature at which a combustible substance begins combustion in the air is called its ignition temperature (ignition point). Let us do Activity 8.12 to compare the ignition temperatures of several combustible substances. Science | Changes in Matter 115 Activity 8.12 You will need:- A tin lid, a stand, a match, a piece of paper, cotton wool, magnesium ribbon, sugar, a piece of sulphur Method:- ² Fix the tin lid to the stand. ² Place the above substances on the tin lid. ² Keep the Bunsen burner underneath the tin lid and heat. ² Observe the sequence in which the combustible substances placed on the tin lid ignite and note it down. The substances which ignite early have low ignition temperature. A combustible substance starts to burn after it gets heated to its ignition point. Thus, three main factor essential for combustion can be identified. They are; Presence of a combustible substance Having access to a supporter of combustion (Oxygen). Heating the combustible substance to its ignition temperature. Fire triangle Pay your attention to a fire broken out by accident. The fire should be extinguished to prevent damage. If a fire is to be extinguished the factors causing fire should be removed Ox at from the fire. The following figure which He yg shows the relationship among the factors en required to create a fire is known as the fire triangle. Examine it well. To extinguish a fire it is required to prevent Fuel the access of the supporter of combustion to Figure 8.8 Fire triangle the fire, prevent reaching the ignition temperature (i.e.prevent receiving heat) and remove the combustible substance. The method we use to extinguish a fire mostly is throwing water over the burning material. In addition to this covering the burning substance with sand and wet gunnies is also done. 116 Science | Changes in Matter ² When water is sprayed over the fire it is extinguished. This is because when water vaporises absorbing heat from the burning material, temperature of under falls below the ignition temperature. ² When somebodys, clothes catch fire, the most suitable method to extinguish it is to roll on the ground. This helps break the connection between air, the supporter of combustion, and the material that has caught fire. When the clothes are on fire you should never run. During running more and more oxygen is supplied to the fire, so it spreads faster. The same method cannot be used to extinguish all fires. The nature of the fire should be identified and then the appropriate method should be selected. Fuels Fuels are substances used to generate heat energy and light energy by combustion. ² Examples for solid fuels :- Firewood, coconut husks, coconut shells, wax ² Examples for liquid fuels :- Kerosine, petrol, diesel, coconut oil ² Examples for gaseous fuels :- Liquid petrolium gas (LP gas), coal gas, methane (bio gas) Almost every fuel contains the elements carbon and hydrogen. Let us carry out Activity 8.13 to identify the products formed during the combustion of fuels. Activity 8.13 Aspirator You will need:- A candle, a boiling tube, a bottle, a funnel, lime water, copper sulphate Water Method:- ² Arrange the apparatus as shown in Figure 8.9. Connect the boiling tube/bottle with lime water to the aspirator. Light Anhydrous Lime the candle and operate the Candle copper sulphate water aspirator. When the aspirator Figure 8.9 works an air current is drawn through the apparatus from the funnel to the boiling tube. Science | Changes in Matter 117 U tube contains anhydrous copper sulphate (white). The boiling tube/bottle contains colourless lime water. When the candle is lit and the aspirator is set to work you will observe that white anhydrous copper sulphate turns blue. Also, it can be seen that the lime water turns milky. White anhydrous copper sulphate turns blue because of the water (water vapour) drawn into the U tube. Lime water turns milky due to carbon dioxide gas. This activity indicates that when a candle burns, water and carbon dioxide gas are produced. Thus, in the combustion of fuels water and carbon dioxide gas are produced as the products. Complete combustion and incomplete combustion of fuels Complete combustion occurs when an adequate oxygen gas (supporter of combustion) is supplied for combustion. You know that fuels contain the elements carbon and hydrogen. On complete combustion carbon gives carbon dioxide and hydrogen gives water. More heat is produced by complete combustion. The combustion occurring in an inadequate supply of oxygen is called incomplete combustion. In this carbon monoxide and unburnt carbon particles are also produced in addition to carbon dioxide and water. In incomplete combustion, the quantity of heat produced by the flame is relatively low. Candle flame Outer zone (barely When a candle is lit, solid wax turns into liquid visible) wax. Liquid wax moves up through the wick and vapourises. This wax vapour, reacts with Luminous oxygen and produces heat and light giving rise zone to the flame of the candle. Observe the candle flame well. It has three clearly visible zones. Non-luminous zone The inner zone is the non-luminous zone. It contains wax vapour. Its temperature is low Blue zone relatively to that of the other zones. Outer to the non-luminous zone is the luminous zone. The unburnt carbon particles present in that zone becomes incandescent emitting a yellow light. The temperature in this zone is greater than that of the non-luminous zone. Outer Figure 8.10 Candle flame to the luminous zone is another zone which appears in blue colour at the base of the flame but is hardly visible in other areas. This is known as the outer zone (invisible zone) and has the highest temperature. 118 Science | Changes in Matter Invisible zone Bunsen flame Light blue zone The colour of the bunsen flame changes with the amount of oxygen gas supplied for combustion. When Dark blue zone the oxygen supply decreases the flame turns yellow Non-luminous and when the flame receives enough oxygen it turns zone blue. By observing the blue flame well, several zones Stem of it can be identified. At the centre of it is the non-luminous zone consisting of unburnt gas. Outer to the non-luminous zone lies a Collar dark blue zone and a light blue zone respectively. The Air pore outerpart is the invisible zone. In the invisible zone Gas tube complete combustion occurs. Foot Figure 8.11 Bunsen flame 8.5 Tarnishing of metals You have learnt that having a shiny surface is a property of metals. When metals are exposed to air for a long period, that lustre disappears. The change in surface of metals like this is called tarnishing. Almost every metal tarnishes. A substance called rust is formed on the surface of iron due to tarnishing. This is reddish brown in colour and is called iron rust. This process is called rusting of iron. Due to tarnishing and rusting the surfaces of metals corrode. This is called corrosion of metals. Tarnishing of metals and rusting of iron are chemical changes. Rusting of iron Let us do Activity 8.14 and Activity 8.15 to investigate the factors essential for rusting of iron. Activity 8.14 You will need:- Three test tubes, cleaned iron nails, coconut oil Method:- ² Take some water into a test tube and heat to boiling. Put a cleaned iron nail into it and cover the water surface with a layer of oil (setup A). Oil layer is placed to prevent the dissolving of air when water cools. Science | Changes in Matter 119 A B C ² Take equal volumes of cold water to two other test tubes and put a cleaned iron nail into each. Put an oil layer to Coconut one of them (set-up B). oil ² Leave the other test tube as it is Boiled Cold (set-up C). water water ² After a few days observe the setups. ² Record your observations. Cleaned iron nails Figure 8.12 The nail in test tube A does not rust. As it contains boiled water all the air dissolved in it has been expelled. Putting a layer of coconut oil on water has prevented the dissolving of air when water cools. Test tube B contains cold water. Therefore, its water contains air. Because there is air dissolved in water the nail in it rusts. The nail in the test tube C is open to the outer environment. As it receives air from outside rusting occurs. Hence it can be concluded that air is essential for rusting. Activity 8.15 Wax You will need:- A boiling tube, two cleaned iron nails, Cork stopper cork stopper, calcium chloride crystals, wax, coconut Iron nails oil Method:- ² Clean the two iron nails with sand paper. ² Fix them to the cork stopper as shown in the Figure 8.13. ² Add calcium chloride crystals to the boiling tube and fix the stopper with the iron nails to it. Calcium ² Make the tube air tight with wax. chloride crystals ² Observe this setup for several days. Figure 8.13 ² Record your observations. After a few days it can be seen that the parts of the nails outside the boiling tube have rusted while the parts inside the tube remain without rusting. 120 Science | Changes in Matter Calcium chloride crystals absorb moisture in the air in the boiling tube. Placing wax around the stopper makes the tube air tight and prevents the entry of moisture in air into the tube. As the air inside the tube is free from moisture, the parts of nails inside the tube do not rust. What is expected by driving the two nails into the cork in opposite directions is to ensure that the pointed tip or the flat head of nails have no effect on rusting. Activity 8.16 Iron filings You will need:- A beaker, two test tubes, iron filings, cotton wool Moist cotton Moist cotton wool wool Method:- Tube B ² Take two test tubes. In one Tube A of them (A) trap some moist cotton wool. In the other tube (B), trap a similar plug of moist cotton wool with some iron filings on it. ² Take some water into a beaker and dip the two Water inverted test tubes A and B in water as shown in Figure Figure 8.14 8.14. ² Observe this setup a few days. ² Record the observations. It can be seen that iron filings in tube B have undergone rusting while water has risen up to about one fifth of its height. The percentage of oxygen in air by volume is 21%. That is, nearly 1/5 th of air in a given space is oxygen. If oxygen gas is used up for rusting, 1/5 th of the volume of air contained in space where rusting occurs should have been spent. For the rusting of iron filings in tube B, oxygen gas in the air in that tube is used up. As 1/5 th of the volume of air is oxygen the water level rises to 1/5 th the height of the test tube. From this it is clear that oxygen gas is consumed during rusting. These activities prove us that oxygen and water vapour/water in air are essential for the rusting of iron. Science | Changes in Matter 121 Protection of iron from rusting Iron objects rust only when they are able to come in contact with air and water. Figure 8.15 A galvanised Figure 8.16 A Painted gate bucket You would have seen that paints are applied on objects made of iron such as grills, gates and bridges. Application of paint is a frequently used method to prevent rusting of iron. It prevents iron from coming into contact with air and water. Grease is also applied in machinery made from iron to prevent rusting. You have heard about the galvanized iron items. During galvanizing, zinc metal is applied on objects made of iron. Iron in galvanized items does not rust even if their zinc coat is scratched exposing some of their points to air. Therefore, galvanizing is a very good protective method. Items such as buckets, roofing sheets and iron nails are protected by galvanizing. Application of tin metal is also another method used to protect iron from rusting. The containers of sealed food such as sardine and milk powder, though commonly called 'tins' are vessels made of iron. In them tin is present only as a coating. However, when scratched tin coated vessels rust very fast. 8.6 Neutralisation Recall what you have learnt in grade 7 about acids, bases and neutral substances. Let us do Activity 8.17 to revise facts about them. 122 Science | Changes in Matter Activity 8.17 You will need:- Test tubes, red litmus, blue litmus, pH papers, hydrochloric acid, sodium hydroxide solution, sodium chloride (salt) solution, phenolphthalein Method:- ² Take hydrochloric acid solution, sodium hydroxide solution and sodium chloride (salt) solution separately into three test tubes. ² Test these three solutions with red litmus papers. ² Test these three solutions with blue litmus papers. ² Test these three solutions with pH A B C papers. Hydrochloric acid Sodium Sodium ² Add two drops of phenolphthalein to solution hydroxide chloride these solutions. solution solution ² Record your observations. Figure 8.17 ² A Solution A turns the blue litmus paper into red. Solution A does not change the colour of red litmus.When examined with a piece of pH paper, a value less than 7 is obtained. On addition of phenolphthalein it stays colourless. ² Solution B does not change the colour of blue litmus. It turns red litmus into blue. When tested by a pH paper the pH value is greater than 7. The solution gives a pink colour with phenolphthalein. ² Solution C does not change the colour of blue litmus or red litmus. The colour it gives with the pH paper corresponds to 7. It does not show a colour change with phenolphthalein. From the above observations it can be identified that solution A is acidic, B is basic and C is neutral. Investigating what type of a change occurs when an acid is added to a base You might have heard that milk of magnesia liquid is given to relieve the acidity in stomach. Milk of magnesia is a basic substance. What is the reason for giving a basic substance like this to minimize the affect of an acidic substance? Let us conduct Activity 8.18 to look into this. Science | Changes in Matter 123 Activity 8.18 You will need:- A beaker, a dropping pipette, dilute sodium hydroxide solution, dilute hydrochloric acid, phenolphthalein Method:- ² Pour dilute sodium hydroxide solution to a beaker. Add a few drops of phenolphthalein into it. Then add dilute hydrochloric acid dropwise into it using a dropping pipette and observe the colour change in the solution. ² When the acid is added the pink colour of the solution gradually decreases and at a certain moment the solution becomes colourless. This indicates that when an acid is added to a base, the basic property of the base gradually disappears. a - Sodium hydroxide solution with a few drops of phenolphthalein b - Nutralised to some extent due to the addition of acid c - Totally neutralised solution Figure 8.18 When an acid is added to a base or base is added to an acid, their acidic and basic properties decrease and at a certain point acidic and basic properties totally disappear. This process is called neutralisation. You know that sodium hydroxide is a base and hydrochloric acid is an acid. When these two react sodium chloride and water are formed which are neutral substances. Sodium hydroxide + Hydrochloric acid Sodium chloride + Water This reaction between an acid and a base is a chemical reaction. It is referred to as a neutralisation reaction. Let us now explore about some instances in which we happen to meet acid-base neutralisation in day-to-day life. When acidity in the stomach increases milk of magnesia is administered. Milk of magnesia means the base magnesium hydroxide. This base neutralises the excess hydrochloric acid in the stomach. 124 Science | Changes in Matter The pain caused by bee stings disappear on application of lime. When bees sting, acidic substances are introduced into the skin. Lime is a base. It neutralises the acid. That is why the pain subsides. The wasp sting is basic. Therefore, when an acidic substance such as vinegar or lemon juice is applied, the poison gets neutralised relieving the pain. Lime is applied to acidic soils. Lime which is a base neutralises acids in the soil. Figure 8.19 Summary ² The changes takes place in matter is of two types, physical changes and chemical changes. ² In the case of physical changes the existing nature of matter changes, though its composition remains unchanged. ² The changes in which the composition of matter changes giving rise to new substances are known as chemical changes. ² Rusting of iron, corrosion of metals, combustion nutralisation are examples for chemical changes. ² Changes of state such as fusion, vapourisation, sublimation, condensation and freezing are physical changes. ² Heat change, evolution of a gas, formation of a precipitate, colour change and change in temperature provide evidence for the occurrence of a chemical reaction. ² The substances take part in a reaction are reactants and the substances formed during a reaction are products. ² During chemical reactions, the total mass does not change. That means, the mass of the reactants that took part in the reaction is equal to the mass of the products formed after the reaction. ² The reaction of combustible substances with oxygen is called combustion. ² When many fuels are subjected to complete combustion, carbon dioxide and water are formed. ² During incomplete combustion unburnt carbon and carbon monoxide are also formed in addition to carbon dioxide and water. ² The quality of heat generated during complete combustion is relatively higher than that generated during incomplete combustion. Science | Changes in Matter 125 ² Water/water vapour and oxygen are essential for the rusting of iron. ² Rusting can be prevented by methods such as applying paint, galvanizing and applying grease. ² When an acid reacts with a base, the acidic properties of the acid and the basic properties of the base disappear. ² The chemical reactions between acids and bases are called neutralisation reactions. Exercises 01) Select the correct or best suitable answer for the following questions. 01. Which of the following is not a chemical change? 1. Condensation of steam 2. Burning of magnesium 3. Rusting of iron 4. Tarnishing of metals 02. Which of the following statement is false? 1. Combustion is a chemical reaction. 2. Oxygen is essential for the rusting of iron. 3. Complete combustion gives rise to a yellow flame. 4. It is necessary to heat something to its ignition temperature to burn. 03. The nail in which test tube does not rust after few days ? Oil Oil layer layer Boiled Fresh Boiled Fresh water water water water A B C D 04. Which of the following is not observed when a piece of zinc is placed in a copper sulphate solution? 1. Gradual dissolution of the piece of zinc. 2. Deposition of a reddish brown substance around the piece of Zinc. 3. Slight heating of the solution. 4. Blue colour of the solution remain same. 05. Which of the following does not undergo a chemical change on heating? A-Sulphur B-Magnesium C-Iron 1. Only A 2. Only A and B 3.Only B and C 4.A, B and C 126 Science | Changes in Matter 02. The diagram shows a bunsen flame. a. Name A, B, C and D zones. D b. In which zone complete burning occurs? C c. What is the fuel that burns in a bunsen burner? 03. Milk of magnesia is prescribed as a remedy for the B discomfort caused by increasing acidity of stomach. a. Is milk of magnesia acidic or basic? A b. How do you name the reaction between milk of magnesia and an acid? 04.Give short descriptions for the following phenomena. 1. Slaked lime is added to avoid acidic nature in soil. 2. Iron is protected from rusting by application of paint. 3. You should never run,when your clothes are on fire. Technical Terms Science | Changes in Matter 127 SCIENCE Part - II Grade 8 Educational Publications Department i First Print 2016 Second Print 2017 Third Print 2018 Fourth Print 2019 All Rights Reserved ISBN 978-955-25-0133-3 Published by Educational Publications Department Printed by Neo Graphics (Pvt) Ltd. No.44, Udahamulla Station Road, Gangodawila, Nugegoda. ii The National Anthem of Sri Lanka Sri Lanka Matha Apa Sri Lanka Namo Namo Namo Namo Matha Sundara siri barinee, surendi athi sobamana Lanka Dhanya dhanaya neka mal palaturu piri jaya bhoomiya ramya Apa hata sepa siri setha sadana jeewanaye matha Piliganu mena apa bhakthi pooja Namo Namo Matha Apa Sri Lanka Namo Namo Namo Namo Matha Oba we apa vidya Obamaya apa sathya Oba we apa shakthi Apa hada thula bhakthi Oba apa aloke Apage anuprane Oba apa jeevana we Apa mukthiya oba we Nava jeevana demine, nithina apa pubudukaran matha Gnana veerya vadawamina regena yanu mana jaya bhoomi kara Eka mavakage daru kela bevina Yamu yamu vee nopama Prema vada sema bheda durerada Namo, Namo Matha Apa Sri Lanka Namo Namo Namo Namo Matha iii wms fjuq tl ujlf. orefjda tl ksjfiys fjfikd tl mdge;s tl reêrh fõ wm lh ;=< ÿjkd tneúks wms fjuq fidhqre fidhqßfhda tl f,i tys jefvkd Ôj;a jk wm fuu ksjfia fid¢k isáh hq;= fõ ieug u fu;a lreKd.=fKkS fj

grade-8-science-text-book-61fb9947be91f.pdf

Document Details

Tags

Related

Full Transcript

Upgrade to continue