NCERT Book Class 8 Science Chemical Effects Of Electric Current

CHEMICAL EFFECTS OF ELECTRIC CURRENT

Your elders might have cautioned you against touching an electrical appliance with wet hands. But do you know why it is dangerous to touch an electrical appliance with wet hands? We have learnt earlier that the materials, which allow electric current to pass through them, are good conductors of electricity. On the other hand, materials, which do not allow electric current to pass through them easily, are poor conductors of electricity. In Class VI, we made a tester(Fig.14.1) to test whether a particular material allows the electric current to pass through it or not. Do you recall how the tester helped us in deciding that? We found that metals such as copper and aluminium conduct electricity whereas materials such as rubber, plastic and wood do not conduct electricity. However, so far we have used our tester to test materials which were in solid state. But what about liquids? Do liquids also conduct electricity? Let us find out.

Paheli and Boojho want to remind you that one should not experiment with the electric supply from the mains or a generator or an inverter. Use only electric cells for all the activities suggested here.

14.1 Do Liquids Conduct Electricity?

To test whether a liquid allows electric current to pass through it or not, we can use the same tester (Fig. 14.1) which we made in Class VI. However, replace the cell by a battery. Also, before usin  the tester we should check whether it isworking or not.

Activity 14.1

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 meansthat 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 withanother bulb. Now test if the tester is working or not. If it is still not working then replace the cells with fresh cells.

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. A tester is used to check the conduction of electricity through two liquids, labeled 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.

3. Does pure water conduct electricity? If not, what can we do to make it conducting?

4. 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.

5. 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?

6. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.

7. 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?

8. Prepare a list of objects around you that are electroplated.

9. The process that you saw in Activity 14.7 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 battery and why?

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 the Activity 14.7 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 14.7). 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 you would 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?

Please refer to attached file for NCERT Class 8 Science Chemical Effects Of Electric Current