Class 8 Science

Chemical Effects of Electric Current

NCERT Solution

Question 1: Fill in the blanks:

  1. Most liquids that conduct electricity are solutions of, ______________ and ______________.
  2. The passage of an electric current through a solution causes ______________ effects.
  3. If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the ___________ terminal of the battery.
  4. The process of depositing a layer of any desired metal on another material by means of electricity is called _________.

Answer: (a) acid, base and salts, (b) chemical, (c) negative, (d) electroplating

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

Answer: Magnetic needle shows deflection because of conduction of electricity through the solution. We know that when electric current passes through a conductor, the conductor behaves like a magnet. This is called magnetic effect of electric current.

Question 3: Name three liquids, which when tested in the manner shown in the figure, may cause the magnetic needle to deflect.

setup for electrochemistry

Answer: Acid solution, salt solution, solution of a base.

Question 4: The bulb does not glow in the setup shown in the figure. List the possible reasons. Explain your answer.

setup for electrochemistry

Answer: Bulb does not glow in the above setup because solution taken is poor conductor of electricity. Solution which is poor conductor of electricity does not allow much current to pass through it which can produce the required amount of heat to make the bulb glowing.

Question 5: 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

  1. liquid A is a better conductor than liquid B.
  2. liquid B is a better conductor than liquid A.
  3. both liquids are equally conducting.
  4. conducting properties of liquid cannot be compared in this manner./li>

Answer: (a) liquid A is better conductor than liquid B

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

Answer: Pure water does not conduct electricity. By dissolving some salt, such as common salt, pure water can be made a conductor of electricity.

Question 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.

Answer: Normal water is good conductor of electricity. Therefore, firemen shut off the main electrical supply for the area before using of water hoses in the case of fire to prevent them from getting electric shock.

Question 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?

Answer: Since, seawater contains more salts dissolved in it, thus in the case of seawater, compass needle of the tester deflects more than the drinking water.

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

Answer: Rainwater may contain many impurities which get dissolved in it when it passes through various layers of troposphere. Due to this, rainwater becomes good conductor of electricity. If an electrician carries out electrical repairs outdoors during heavy downpour, he may get electric shock. Hence, it is not safe for the electrician to carry out electrical repairs outdoors during heavy downpour.

Question 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?

Answer: Many impurities get dissolved in the rain water while it comes to the ground. Due to the presence of these impurities, rainwater conducts electricity. This explains Paheli’s observation.

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

Answer: Rim of cycle, handle of cycle, electric iron, door handles, water tap, water pipe.

Question 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 battery and why?

Answer: Electrode of impure copper is connected with the negative terminal. Because pure copper metal, which is positively charged, from positive terminal is deposited over the negative terminal of thin copper after dissolving in the solution.