Chemical Effects of Electric Current PDF

CHEMICAL EFFECTS OF ELECTRIC CURRENT Y our elders might have cautioned plastic and wood do not conduct you against touching an electrical electricity. However, so far we have used appliance with wet hands. But...

CHEMICAL EFFECTS OF ELECTRIC CURRENT Y our elders might have cautioned plastic and wood do not conduct you against touching an electrical electricity. However, so far we have used appliance with wet hands. But our tester to test materials which were do you know why it is dangerous to in solid state. But what about liquids? touch an electrical appliance with wet Do liquids also conduct electricity? Let hands? us find out. We have learnt earlier that the materials, which allow electric current to pass through them, are good Paheli and Boojho want to conductors of electricity. On the other remind you that one hand, materials, which do not allow should not experiment electric current to pass through them with the electric supply easily, are poor conductors of electricity. from the mains or a In Class VI, we made a tester to test generator or an inverter. whether a particular material allows the Use only electric cells for electric current to pass through it or not. all the activities suggested here. Do you recall how the tester helped us in deciding that? We found that metals such as copper 11.1 Do Liquids Conduct and aluminium conduct electricity Electricity? whereas materials such as rubber, To test whether a liquid allows electric current to pass through it or not, we can use the same tester (Fig.11.1). Fig.11.1 : A tester 2024-25 However, replace the cell by a battery. of the tester glow? Does lemon juice Also, before using the tester we or vinegar conduct electricity? How should check whether it is working would you classify lemon juice or or not. vinegar— a good conductor or a poor conductor? Activity 11.1 Join the free ends of the tester together for a moment. This completes the circuit of the tester and the bulb should glow. However, if the bulb does not glow, it means that the tester is not working. Can you think of the possible reasons? Is it possible that the connections are loose? Or, the bulb is fused? Or, your cells are used up? Check that all the connections are tight. If they are, then replace the bulb with another bulb. Now test if the tester is working or not. If it is still not Fig. 11.2 : Testing conduction of electricity in working then replace the cells with lemon juice or vinegar fresh cells. When the liquid between the two Now that our tester is working, let ends of the tester allows the electric us use it to test the various liquids. current to pass, the circuit of the tester (Caution: While checking your tester, becomes complete. The current flows in the circuit and the bulb glows. When do not join its free ends for more than a the liquid does not allow the electric few seconds. Otherwise the cells of the current to pass, the circuit of the tester battery will drain very quickly.) is not complete and the bulb does not glow. Activity 11.2 In some situations even though the Collect a few small plastic or rubber liquid is conducting, the bulb may not caps of discarded bottles and clean g l o w. It may have happened in them. Pour one teaspoon of lemon Activity 11.2. What can be the reason? juice or vinegar in one cap. Bring Do you remember why the bulb your tester over this cap and let the glows when the electric current passes ends of the tester dip into lemon juice through it? Due to the heating effect or vinegar as shown in Fig.11.2. Take of current, the filament of the bulb gets care that the ends are not more than heated to a high temperature and it 1 cm apart but at the same time do starts glowing. However, if the current not touch each other. Does the bulb through a circuit is too weak, the filament does not get heated CHEMICAL EFFECTS OF ELECTRIC CURRENT 139 2024-25 sufficiently and it does not glow. And Activity 11.3 why is the current in the circuit weak? Well, though a material may conduct Take the tray from inside a discarded electricity, it may not conduct it as matchbox. Wrap an electric wire a few easily as a metal. As a result, the times around the tray. Place a small circuit of the tester may be complete compass needle inside it. Now and yet the current through it may be connect one free end of the wire to too weak to make the bulb glow. Can the terminal of a battery. Leave the we make another tester which can other end free. Take another piece of detect a weak current? wire and connect it to the other terminal of the battery (Fig. 11.4). We can use another effect of an electric current to make another kind of tester. Do you recall that electric current produces a magnetic effect? What happens to a compass needle kept nearby when current flows in a wire? Even if the current is small, the deflection of the magnetic needle can be seen. Can we make a tester using the magnetic effect of currents? Let us find out in Activity 11.3. You may use an LED (Fig. 11.3) in Fig 11.4 : Another tester place of the electric bulb in the tester of Fig. 11.2. LED glows even when a Join the free ends of two wires weak electric current flows through momentarily. The compass needle it. should show deflection. Your tester There are two wires (called leads) with two free ends of the wire is ready. attached to an LED. One lead is Now repeat Activity 11.2 using this slightly longer than the other. tester. Do you find a deflection in the Remember that while connecting to compass needle the moment you dip a circuit, the longer lead is always the free ends of the tester in lemon connected to the positive terminal of juice? the battery and the shorter lead is Take out the ends of the tester connected to the negative terminal of the battery. from the lemon juice, dip them in water and then wipe them dry. Repeat the activity with other liquids such as tap water, vegetable oil, milk, honey. (Remember to wash and wipe dry the ends of tester after testing each liquid). In each case observe whether the magnetic needle shows deflection or not. Record your Fig. 11.3 : LEDs observations in Table 11.1. 140 SCIENCE 2024-25  Table 11.1 : Good/Poor Conducting Liquids S.No. Material Compass Needle Shows Good Conductor/ Deflection Yes/No Poor Conductor 1. Lemon juice Yes Good Conductor 2. Vinegar 3. Tap Water 4. Vegetable oil 5. Milk 6. Honey 7. 8. 9. 10. From Table 11.1, we find that some We have tested the conduction of liquids are good conductors of electricity electricity through tap water. Let us now and some are poor conductors. test the conduction of electricity through distilled water. When the free ends of the Activity 11.4 tester do not touch each Take about two teaspoonfuls of other, there is an air gap distilled water in a clean and dry between them. Paheli knows plastic or rubber cap of a bottle. (You that air is a poor conductor of may obtain distilled water from your electricity. But she has also read that during lightning, an electric school science lab. You may also get current passes through air. She distilled water from a medical store wonders if air is indeed a poor or a doctor or a nurse). Use the tester conductor under all conditions. to test whether distilled water This makes Boojho ask whether conducts electricity or not. What do other materials classified as you find? Does distilled water poor conductors also allow conduct electricity? Now dissolve a electricity to pass under pinch of common salt in distilled certain conditions. water. Again test. What do you conclude this time? Actually, under certain conditions When salt is dissolved in distilled most materials can conduct. That is why water, we obtain salt solution. This is a it is preferable to classify materials as conductor of electricity. good conductors and poor conductors The water that we get from sources instead of classifying as conductors and such as taps, hand pumps, wells and insulators. ponds is not pure. It may contain CHEMICAL EFFECTS OF ELECTRIC CURRENT 141 2024-25 several salts dissolved in it. Small Most liquids that conduct electricity amounts of mineral salts are naturally are solutions of acids, bases and salts. present in it. This water is thus a good When an electric current flows conductor of electricity. On the other through a conducting solution, does it hand, distilled water is free of salts and produce an effect on the solution? is a poor conductor. 11.2 Chemical Effects of Electric Current Small amounts of mineral salts present naturally in water are In Class VII, we have learnt some effects beneficial for human health. of electric current. Can you list these However, these salts make water effects? What effect does the current a good conductor. So, we should produce when it flows through a never handle electrical conducting solution? Let us find out. appliances with wet hands or while standing on a wet floor. Activity 11.6 We have found that common salt, Take out carbon rods carefully from when dissolved in distilled water, makes two discarded cells. Clean their it a good conductor. What are the other metal caps with sand paper. Wrap substances which, when dissolved in copper wires around the metal caps distilled water, make it conducting? Let us find out. of the carbon rods and join them to a battery (Fig. 11.5). We call these Caution: Do the next activity under the two rods electrodes. (Instead of supervision of your teacher/parent or some elderly person, because the use of acid is involved in it. Activity 11.5 Take three clean plastic or rubber caps of bottles. Pour about two teaspoonfuls of distilled water in each of them. Add a few drops of lemon juice or dilute hydrochloric acid to distilled water in one cap. Now in the second cap containing distilled water, add a few Metal Cap drops of a base such as caustic soda or potassium iodide. Add a little sugar Carbon Carbon to the distilled water in the third cap rod rod and dissolve it. Test which solutions conduct electricity and which do not. Water What results do you obtain? Fig.11.5 : Passing current through water 142 SCIENCE 2024-25  carbon rods, you may take two iron nails about 6 cm long.) Pour a cupful of water in a glass/plastic Boojho decided to test bowl. Add a teaspoonful of salt or a whether some fruits and vegetables also conduct few drops of lemon juice to water to electricity or not. He cut make it more conducting. Now a potato into two halves and immerse the electrodes in this inserted the copper wires of a tester solution. Make sure that the metal into it. Just then his mother called caps of the carbon rods are outside him and he forgot to take out the the water. Wait for 3-4 minutes. wires of the tester inserted into the Observe the electrodes carefully. Do potato. When he came back after you notice any gas bubbles near the half an hour, he noticed that there was a greenish blue spot on the electrodes? Can we call the change potato around one wire whereas taking place in the solution a there was no such spot around the chemical change? Recall the other wire (Fig. 11.6). definition of a chemical change that you learnt in Class VII. In 1800, a British chemist, William Nicholson (1753–1815), had shown that if electrodes were immersed in water, and a current was passed, bubbles of oxygen and hydrogen were produced. Oxygen bubbles formed on the electrode Fig. 11.6 : Testing potato connected to the positive terminal of He was surprised with this the battery and hydrogen bubbles observation and along with Paheli formed on the other electrode. repeated this activity many times. They The passage of an electric current found that it was always the wire through a conducting solution causes connected to the positive terminal, which had a greenish blue spot around chemical reactions. As a result, bubbles it. They felt that this discovery was very of a gas may be formed on the electrodes. useful because it could be used for Deposits of metal may be seen on identifying the positive terminal of a electrodes. Changes of colour of cell or a battery concealed in a box. solutions may occur. The reaction would They decided to report their finding to depend on what solution and electrodes a children’s magazine. are used. These are some of the chemical Remember that Boojho set out to effects of the electric current. test whether potato conducted CHEMICAL EFFECTS OF ELECTRIC CURRENT 143 2024-25  electricity or not. What he found was that current produced a chemical effect in the potato. To him this was very exciting. In fact, this is how science sometimes works. You are looking for something and you discover something else. Many Copper plate important discoveries have been made in this manner. Copper plate Copper sul- 11.3 Electroplating phate solution You know that a brand new bicycle has shiny handlebar and wheel rims. Fig.11.7 : A simple circuit showing However, if these are accidentally electroplating scratched, the shiny coating comes off Allow the current to pass for revealing a not so shiny surface beneath. about 15 minutes. Now remove the You might have also seen women using electrodes from the solution and ornaments, which appear to be made of look at them carefully. Do you find gold. However, with repeated use, the any difference in any one of them? gold coating wears off, revealing silver or Do you find a coating over it? What some other metal beneath. colour is the coating? Note down the In both these cases, a metal has a terminal of the battery with which coating of another metal. Do you wonder this electrode is connected. how a layer of one metal can be deposited on top of another? Well, let us try doing it ourselves. After doing the electroplating activity, Paheli interchanged Activity 11.7 the electrodes and repeated We will need copper sulphate and the activity. What do you think she would observe this time? two copper plates of size around 10 cm × 4 cm. Take 250 mL of distilled water in a clean and dry When electric current is passed beaker. Dissolve two teaspoonfuls of through the copper sulphate solution, copper sulphate in it. Add a few drops copper sulphate dissociates into of dilute sulphuric acid to copper copper and sulphate. The free copper sulphate solution to make it more gets drawn to the electrode connected conducting. Clean copper plates with to the negative terminal of the battery sand paper. Now rinse them with and gets deposited on it. But what water and dry them. Connect the about the loss of copper from the copper plates to the terminals of a solution? battery and immerse them in copper From the other electrode, a copper sulphate solution (Fig. 11.7). plate, an equal amount of copper gets dissolved in the solution. Thus, the 144 SCIENCE 2024-25 loss of copper from the solution is restored and the process continues. This means that copper gets transferred from one electrode to the other. Boojho could get only one copper plate. So he performed Activity 11.7 by connecting a carbon rod in place of the copper plate which was connected to the negative terminal of the battery. He succeeded in obtaining a coating of copper on carbon rod. The process of depositing a layer of any desired metal on another material by means of electricity is called Fig. 11.8 : Some electroplated objects electroplating. It is one of the most silver and gold on less expensive metals. common applications of chemical effects These ornaments have the appearance of electric current. of silver or gold but are much less Electroplating is a very useful expensive. process. It is widely used in industry Tin cans, used for storing food, are for coating metal objects with a thin made by electroplating tin onto iron. Tin layer of a different metal (Fig.11.8). The is less reactive than iron. Thus, food layer of metal deposited has some does not come into contact with iron and desired property, which the metal of the is protected from getting spoilt. object lacks. For example, chromium Iron is used in bridges and plating is done on many objects such automobiles to provide strength. as car parts, bath taps, kitchen gas However, iron tends to corrode and rust. burners, bicycle handlebars, wheel rims So, a coating of zinc is deposited on iron and many others. to protect it from corrosion and Chromium has a shiny appearance. formation of rust. It does not corrode. It resists scratches. However, chromium is expensive and it In the electroplating factories the may not be economical to make the disposal of the used conducting whole object out of chromium. So the solution is a major concern. It is a object is made from a cheaper metal and polluting waste and there are specific only a coating of chromium over it is disposal guidelines to protect the environment. deposited. Jewellery makers electroplate CHEMICAL EFFECTS OF ELECTRIC CURRENT 145 2024-25 KEYWORDS WHAT YOU HAVE LEARNT ELECTRODE Ü Some liquids are good conductors of electricity ELECTROPLATING and some are poor conductors. Ü Most liquids that conduct electricity are GOOD CONDUCTOR solutions of acids, bases and salts. Ü The passage of an electric current through a LED conducting liquid causes chemical reactions. POOR CONDUCTOR The resulting effects are called chemical effects of currents. Ü The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating. Exercises 1. Fill in the blanks. (a) Most liquids that conduct electricity are solutions of , ______________ and ______________. (b) The passage of an electric current through a solution causes ______________ effects. (c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the terminal of the battery. (d) The process of depositing a layer of any desired metal on another material by means of electricity is called. 2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason? 3. Name three liquids, which when tested in the manner shown in Fig.11.9, may cause the magnetic needle to deflect. Fig. 11.9 146 SCIENCE 2024-25  4. The bulb does not glow in the setup shown in Fig.11.10. List the possible EXERCISES reasons. Explain your answer. Fig. 11.10 5. A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that (i) liquid A is a better conductor than liquid B. (ii) liquid B is a better conductor than liquid A. (iii) both liquids are equally conducting. (iv) conducting properties of liquid cannot be compared in this manner. 6. Does pure water conduct electricity? If not, what can we do to make it conducting? 7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this. 8. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason? 9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain. 10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise she found that the compass needle showed deflection. What could be the reasons? 11. Prepare a list of objects around you that are electroplated. 12. The process that you saw in Activity 11.7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transfered to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why? CHEMICAL EFFECTS OF ELECTRIC CURRENT 147 2024-25  Extended Learning — Activities and Projects 1. Test the conduction of electricity through various fruits and vegetables. Display your result in a tabular form. 2. Repeat Activity 11.7 with a zinc plate in place of the copper plate connected to the negative terminal of the battery. Now replace zinc plate with some other metallic object and again repeat the activity. Which metal gets deposited over which other metal? Discuss your findings with your friends. 3. Find out if there is a commercial electroplating unit in your town. What objects are electroplated there and for what purpose? (The process of electroplating in a commercial unit is much more complex than what we did in Activity 11.7). Find out how they dispose off the chemicals they discard. 4. Imagine that you are an ‘entrepreneur’ and have been provided a loan by a bank to set up a small electroplating unit. What object would you like to electroplate and for what purpose? (Look up the meaning of ‘entrepreneur’ in a dictionary). 5. Find out the health concerns associated with chromium electroplating. How are people trying to resolve them? 6. You can make a fun pen for yourself. Take a conducting metal plate and spread a moist paste of potassium iodide and starch. Connect the plate to a battery as shown in Fig. 11.11. Now using the free end of the wire, write a few letters on the paste. What do you see? Fig. 11.11 148 SCIENCE 2024-25  For more information on this topic visit: l electronics.howstuffworks.com/led.htm Did You Know? LEDs (Light Emitting Diodes) are available in many colours such as red, green, yellow, blue, white and are increasingly being used for many applications, for example in traffic signal lights. LEDs are increasingly being used for lighting. A cluster of white LEDs grouped together forms a LED light source. LED light sources consume less electricity and have longer lifetime than light bulbs and fluorescent tubes. Hence these are gradually becoming the preferred lighting source. CHEMICAL EFFECTS OF ELECTRIC CURRENT 149 2024-25