NCERT Class 8 Science Chapter 14 - Chemical Effects of Electric Current

Chapter 14 - Chemical Effects of Electric Current

Question 1: Define
a) Electrode - Electrodes are the terminals through which current is passed in a liquid.

b) LED - LED is Light Emitting Diode. The longer lead is always connected to the positive terminal of the battery and the shorter lead is connected to the negative terminal of the cell.

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: The deflection in magnetic needle shows that the circuit is complete and the solution conducts electricity meaning that the solution is a good conductor of electricity.

Question 3: Name three liquids, which when tested in the manner shown in Fig. 14.5, may cause the magnetic needle to deflect.

Answer: Vinegar, lemon juice hydrochloric acid etc.

Question 4: The bulb does not glow in the setup shown in Fig. 14.6. List the possible reasons. Explain your answer.

Answer:
The bulb may not glow as
• The wires in the circuit may be loosely connected.
• The bulb may be fused.
• The liquid may be a poor conductor of electricity.
• The cells may be used up.

Question 5: Define
a) Conductor - Substances which allow electric current to pass through them easily are called Conductor. Example: silver, gold, aluminium, etc.

b) Insulator - Substances which don’t allow electric current to pass through them are called Insulator. Example: plastic, rubber, glass, etc.

Question 6: Does pure water conduct electricity? If not, what can we do to make it conducting?
Answer: No, pure water doesn’t conduct electricity. If we dissolve some amount of salt in pure water, then it conducts 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: The water used in the water hoses conducts electricity. So, the firemen shuts off the electric supply before spraying water to save themselves and other people from electrocution.

Question 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 explain the reason?
Answer: The seawater contains a large amount of salt in comparison to drinking water. As a result the seawater is a better conductor of electricity than drinking water. Therefore, the compass needle deflects more in case of seawater.

Question 9: Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Answer: No, it is very dangerous for an electrician to carry out the electrical repairs outdoors during the heavy downpour. It can cause electrocution, as water is a good conductor of electricity.

Question 10: Define
a) Electroplating - The process of depositing a layer of any desired material on another material by means of electricity is called electroplating.

Question 11: 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: 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.

Question 12: Prepare a list of objects around you that are electroplated.
Answer: Some objects that are electroplated include handles of bike and car, jewellery, tin cans etc.

Question 13: 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?
Answer: The thick rod of impure copper plate is to be attached to the positive terminal of the battery because when electric current is passed through the copper sulphate solution, it gets dissociated into copper and sulphate. The free copper, being positively charged, gets drawn to the negative terminal of the battery and gets deposited on it. On the other hand the loss of copper from the solution is regained from the impure copper rod which is attached to the positive terminal of the battery.