NCERT solutions for class 8 Science Chapter 14 Chemical Effects Of Electric Current

Solution for Exercise

Q.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 _______


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Answer :

(a) Most liquids that conduct electricity are solutions of __acids__, __bases__ and __salts__. (b) The passage of an electric current through a solution causes __chemical__ effects. (c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the __negative__ terminal of the battery. (d) The process of depositing a layer of any desired metal on another material by means of electricity is called __electroplating__.

Q.2 When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?


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Answer :

The deflection in the compass needle shows that current is flowing through the wounded wire and hence, through the circuit. The circuit is complete since free ends of the tester are dipped in a solution. The solution is certainly a conducting solution. This is the reason why the compass needle shows a deflection.

Q.3 Name three liquids, which when tested in the manner shown in Fig, may cause the magnetic needle to deflect.


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Answer :

Liquids like lemon juice, salt water and vegetable oil allow electricity to pass through them. Hence, these liquids can be used as in the beaker to show the given effect.

Q.4 The bulb does not glow in the setup shown in given Fig List the possible reasons. Explain your answer.


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Answer :

The bulb may not glow because of the following reasons:
(i) The cells may be used up.
(ii) The liquid may be an insulator, i.e., a poor conductor of electricity.

Q.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.


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Answer :

(i) liquid A is a better conductor than liquid B.

Q.6 Does pure water conduct electricity? If not, what can we do to make it conducting?


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Answer :

No, pure water does not conduct electricity. If we add some salt in water then water will be conductor of electricity

Q.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.


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Answer :

Water may conduct electricity. If the electrical supply for the area is not shut off and water is poured over electrical appliances, then electricity may pass through water and harm the firemen. That is why, in case of a fire, the firemen shut off the main electrical supply for the area before they use the water hoses.

Q.8 A child staying in a coastal region test 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 ex¬plain the reason?


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Answer :

The seawater contains a huge amount of salts in comparison to drinking water, hence the seawater is a better conductor of electricity and it produces a stronger magnetic field in the wire and hence deflects the compass needle more.

Q.9 Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.


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Answer :

No. It is not safe to repair electrical appliances outdoors during heavy downpour. This is because rain water contains dissolved salts. Therefore, rain water can conduct electricity. The electrician may get electrical shocks while working outdoors during rain.

Q.10 Paheli had heard that rain water is as good as distilled water. So, she collected some rain water 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?


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Answer :

Rainwater is pure water which is an insulator but it gets mixed with air pollutants like sulphur dioxide and nitrogen oxides and form acidic solution, which is a good conductor of electricity. So, the compass needle showed deflection..

Q.11 Prepare a list of objects around you that are electroplated.


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Answer :

Examples of electroplated objects are as follows:
(i) gold plating: A fine layer of gold is deposited on the silver ornaments and they are called gold-plated ornaments.
(ii) Zinc plating: Iron used in constructing a building is coated with a layer of zinc. This protects iron from corrosion and rusting.
(iii) Chromium plating: This is done on exterior parts of automobiles in order to obtain a shiny appearance.

Q.12 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 transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?


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Answer :

Copper ion is positively charged. It is attracted towards the plate which is connected to the negative terminal of the battery. As copper ions are transferred to the thin copper plate, this thin pure copper plate must be connected to the negative terminal of the battery. Consequently, impure copper rod is connected to the positive terminal of the battery.



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