NCERT Solutions for Class 8 Science Chapter 14 : Chemical Effects of Electric Current

Q:

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

A:

The deflection in the compass needle proves that current is flowing through the circuit. It shows that the solution is a conducting solution.

Q:

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?

A:

Rain water acts as conducting solution as it contains dissolved salts. On the other hand, there are no dissolved salts present in the distilled water. Therefore, rain water can allow electricity to pass through it.

Q:

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

A:

No, During heavy downpour, it is not safe to repair electrical appliances outdoors. Rain water can conduct electricity as it contains dissolved salts. Hence, the electrician may get electrical shocks while working outdoors during rain.

Q:

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?

A:

Sea water is more salty than the drinking water. Therefore, it is more conducting than the drinking water. Due to this, the compass needle deflects more in the case of seawater than in the drinking water.

Q:

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.

A:

Water can act as a conductor of electricity. In case of a fire, if the electrical supply for the area is not shut off and water is poured over electrical appliances, then the fireman can get electric shocks.

Q:

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

A:

No,Pure water does not conduct electricity because it does not contain salt. When some common salt is added to the pure water, it can conduct electricity.

Q:

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.

A:

The correct option is (i).

Explanation: The magnitude of current flowing through a conducting solution depends on the conductivity of the solution. The solution having more conductivity will allow more current to pass through it.

Q:

The bulb does not glow in the setup shown in Fig.14.10. List the possible reasons. Explain your answer.

A:

There may be two reasons for this.

(i) The battery connected in the circuit has insufficient energy to generate electricity.
(ii) Liquid kept in the beaker is non-conducting. In this case, current would not be able to pass through the liquid.

Q:

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 the battery and why?

A:

Copper ions have positive charges. Hence, they are 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 should be connected to the negative terminal of the battery. As a result, impure copper rod should be connected to the positive terminal of the battery.

Q:

Prepare a list of objects around you that are electroplated.

A:

  1. In gold-plated ornaments, a fine layer of gold is deposited on the silver ornaments.
  2. Chromium plating is done on different parts of vehicles to make them shiny.
  3. To protect iron from corrosion, iron is coated with zinc layer.

Q:

Name three liquids, which when tested in the manner shown in Fig.14.9, may cause the magnetic needle to deflect.

A:

The liquids such as lemon juice, salt water and vegetable oil allow electricity to pass through them. Therefore, these liquids can be used in the beaker shown here.

Q:

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 ______________.

A:

(a) Most liquids that conduct electricity are solutions of acid, bases and salts.
     Explanation: The solutions of acids, bases or salts can conduct electricity.


(b) The passage of an electric current through a solution causes chemical effects.
     Explanation: The solution decomposes into positive and negative ions when electric current passes through it.

(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the negative terminal of the battery.
     Explanation: On passing an electric current through a copper sulphate solution, the solution decomposes into positively charged copper ions and negatively charged sulphate ions. The positively charged copper ions get attracted towards the plate which is 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.

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