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

Chemical Effects of Electric Current

Question 1.
Fill in the blanks.

(a) Most liquids that conduct electricity are solutions of ________ , ________ and ________.
Answer:

  1. Acids
  2. Bases
  3. Salts

(b) The passage of an electric current through a solution causes ________ effects.
Answer:
Chemical

(c) If you pass current through copper sulfate solution, copper gets deposited on the plate connected to the ________ terminal of the battery.
Answer:
Negative

(d) The process of depositing a layer of any desired metal on another material by means of electricity, is called _________.
Answer:
Electroplating

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

Answer:
Yes, it is because the solution conducts electricity.

Question 3.
Name three liquids, which when tested in this manner shown in Fig., may cause the magnetic needle to deflect.
NCERT Solutions for Class 8 Science img 35

Answer:
Copper sulfate solution, sodium hydroxide solution, and sodium chloride solution.

Question 4.
The bulb does not glow in the set-up shown in Fig. List of possible reasons. Explain your answer.
NCERT Solutions for Class 8 Science img 36

Answer:
List of possible reasons :

  1. The liquid solution might be conducting electricity, but the current produced is too small so that the filament of the bulb does not get heated and the bulb does not glow.
  2. Or, it is possible that the connections are loose.
  3. Or, the bulb is fused.
  4. Or, the cells are used up.

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.

Answer:
1. Liquid A is a 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. It can be made conducting if acid, base, or salt is dissolved in it.

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 to extinguish the fire, have a lot of minerals and dissolved salts, which make it conducting. Before throwing water into the burning houses, the mains of electricity in the area is turned off because then the water may conduct huge current and firemen get electrocuted.

Question 8.
A child staying in the 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:
The impure water is the conductor of electricity. Seawater contains more impurity (salt) than drinking water. Therefore, seawater conducts more electricity than drinking water. That is why the compass needle deflects more in the case of seawater.

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

Answer:
When it rains heavily, the rainwater dissolves many impurities from the atmosphere, which makes it impure and hence conductive. The air too becomes humid with this water and behaves as a conductor to electricity. That is why, it is not wise for an electrician to do electrical repair work when it rains heavily, because he may get a dreadful shock or even get electrocuted.

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 reason?

Answer:
Rainwater is, of course, as good as distilled water but, when it passes through the atmosphere, it dissolves a lot of dust, dirt, and impurities and becomes conductive. So, when Paheli used a tester, its compass showed deflection.

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

Answer:
The objects which are electroplated are :

  1. Taps of water connection,
  2. Parts of the bicycle.
  3. Body (parts) of cars, motorcycles, and tractors.
  4. The handle of the doors, etc.

Question 12.
The process that you saw in Activity 14.7 is used for the 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 should be attached to the positive terminal of the battery. This is because the free copper of the solution will get deposited on the thin rod (connected to the negative terminal of the battery) and the loss of copper from the solution will be restored from the thick rod connected to the positive terminal of the battery. Thus, copper from the thick rod could be extracted out.

InText Questions and Answers

Question 1.
Do you know why it is dangerous to touch an electrical appliance with wet hands?

Answer:
If someone operates electrical appliances with wet hands, there is a chance of getting electrocuted as water is a good conductor of electricity.

Question 2.
Do you recall how the tester helped us in deciding that?

Answer:
If the electric current passes through the tester, its bulb glows. If the bulb does not glow, it means that the electric current is not passing.

Question 3.
Do liquids also conduct electricity?

Answer:
Yes, tap water, lemon juice, and acids are some of the liquids which conduct electricity.

14.1 Do Liquids Conduct Electricity? Activity 14.1

Question 1.
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?

Answer:
The reason can be either the connections of the circuit are loose or the bulb is fused or cells are used up.

Activity 14.2

Question 1.
Does the bulb of the tester glow? Does lemon juice or vinegar conduct electricity? How would you classify lemon juice or vinegar – a good conductor or a poor conductor?

Answer:
Yes, the bulb glows. Yes, lemon juice or vinegar conduct electricity. These can be classified as a good conductor.

Question 4.
In some situations even though the liquid is conducting, the bulb may not glow. It may have happened in Activity 14.2. What can be the reason?

Answer:
If the current through a circuit is too small, the filament of the bulb does not get heated to a high temperature and it does not glow.

Question 5.
Can we make another tester that can detect a weak current?

Answer:
To detect a weak current, we can use a LED as a tester, because LED glows even when a small electric current flows through it.

Question 6.
Do you recall that electric current produces a magnetic effect? What happens to a compass needle kept nearby when current flows in a wire? Even if the current is small, the deflection of the magnetic needle can be seen. Can we make a tester using the magnetic effect of currents?

Answer:
Yes, I remember that an electric current produces a magnetic effect. When a compass needle is kept nearby a wire through which the current is flowing, the needle gets deflected. Yes, we can make a tester using the magnetic effect of currents. Even if the current is small, the deflection of the magnetic needle can be seen.

Activity 14.3

Question 1.
Do you find a deflection in the compass needle, the moment you dip the free ends of the tester in lemon juice?

Answer:
Yes, there is a deflection in the compass needle.

Question 2.
Record your observations in Table 14.1.

Answer:
NCERT Solutions for Class 8 Science img 38

Question 3.
Boojho asks whether other materials classified as poor conductors also allow electricity to pass under certain conditions?

Answer:
Yes, some other materials classified as poor conductors also allow electricity to pass under certain conditions. For example, pure water is an insulator but when a pinch of salt is added to it, it becomes a conductor.

Activity 14.4

Question 1.
Use the tester to test whether distilled water conducts electricity or not. What do you find? Does distilled water conduct electricity?

Answer:
The bulb of the tester does not glow when the tester is put in distilled water. This means that distilled water does not conduct electricity.

Question 2.
Now dissolve a pinch of common salt in distilled water. Again test. What do you conclude this time?

Answer:
When the tester is dipped in the distilled water in which salt has been dissolved, the bulb of the tester glows. This shows that when a pinch of salt is dissolved in distilled water, it conducts electricity.

Question 3.
What are the other substances which when dissolved in distilled water, make it conducting?

Answer:
Acids, bases, and salts, which when mixed in the distilled water, make it conducting.

Activity 14.5

Question 1.
What results do you obtain?

Answer:
We find that all three solutions conduct electricity. It means acids, bases, and salts, when mixed in the distilled water, conduct electricity.

Question 10.
When an electric current flows through a conducting solution, does it produce an effect on the solution?

Answer:
The electricity produces a chemical effect when it flows through a conducting solution.

14.2 Chemical Effects of Electric Current Activity 14.6

Question 1.
Do you notice any gas bubbles near the electrodes?

Answer:
Yes, oxygen and hydrogen gas bubbles are formed near the electrodes.

Question 2.
Can we call the change taking place in the solution a chemical change?

Answer:
Yes, it is a chemical change.

14.3 Electroplating

Question 11.
Do you wonder how a layer of one metal can be deposited on top of another?

Answer:
This can be done by electroplating.

Activity 14.7

Question 1.
Do you find any difference in any one of them (electrodes)?

Answer:
Yes, there is a deposition of copper on one of the electrodes.

Question 2.
Do you find a coating over it?

Answer:
Yes, I can find a coating over the electrode.

Question 3.
What color is the coating?

Answer:
It is brown in color.

Question 4.
Note down the terminal of the battery with which this electrode is connected.

Answer:
The electrode is connected with the negative terminal of the battery.

Question 5.
After doing the electroplating activity, Paheli interchanged the electrodes and repeated the activity. What do you think she would observe this time?

Answer:
She will observe that the thicker plate becoming thinner and the thinner plate becoming thicker due to the movement of copper from a thicker plate to a thinner plate.

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