NCERT Solutions for Class 8 Science Chapter 11 Chemical Effects Of Electric Current

Q1. 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_____________ .

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.

Q2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?

Answer:

The magnetic needle shows deflection and therefore current must be flowing through the circuit. The circuit is complete and therefore the solution is electrically conducting due to the presence of charged particles called ions.

Q3 . Name three liquids, which when tested in the manner shown in Fig.11.9, may cause the magnetic needle to deflect.

Answer:

Three liquids which may cause the magnetic needle to deflect when tested in the manner shown in Fig 10.9 are lemon juice, salt solution and vinegar.

Q4. The bulb does not glow in the setup shown in Fig.11.10 List the possible reasons. Explain your answer.

Answer:

The bulb might not glow due to the following reasons:

(a) The liquid might not be a conductor of electricity.

(b) The current might be very weak and not sufficient enough to light the bulb, this could happen due to the battery not being charged or because of the circuit being poorly conducting.

Q5. 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.

Answer:

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

The brightness of the bulb depends on the current flowing through it which in turn depends on the conductivity of the liquid. Since the brightness of the bulb is more in case of liquid A we conclude liquid A is a better conductor than liquid B.

Q6. Does pure water conduct electricity? If not, what can we do to make it conducting?

Answer:

No, pure water does not conduct electricity because of the absence of charged particles known as ions which facilitate conduction of electricity. To make it conducting we can add salt, acid or base to the water.

Q7. 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:

As we know water is a conductor of electricity, the poured water may come in contact with some electrical appliance or electrical cable causing the current to reach the firemen and harm them. Thus to avoid this the main electrical supply for the area is shut down before the firemen use the water hoses.

Q8. 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:

Seawater is salty and as we know salt imparts the property of electrical conduction to water, it will be more conducting than drinking water which has a lesser amount of dissolved salt than seawater. Due to higher electrical conductivity, the deflection of the needle is more in the case of seawater.

Q9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.

Answer:

No, it is not safe for the electrician to carry out electrical repairs outdoors during a heavy downpour as rainwater contains dissolved salts which makes rainwater a good conductor of electricity and due to this the electrician might get an electric shock while working outdoors during a heavy downpour.

Q10. 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:

The reason behind the needle showing deflection in case of rainwater is the fact that rainwater has dissolved salts in it which makes it a good conductor of electricity whereas distilled water is completely devoid of any salts and does not conduct electricity.

Q11. Prepare a list of objects around you that are electroplated.

Answer:

List of objects which are electroplated around us:

(i) Gold is electroplated on ornaments.

(ii) Zinc is electroplated on pipes and machinery.

(iii) Chromium is electroplated on parts of vehicles.

(iv) Tin is used to coat metal containers for food.

Q12. The process that you saw in Activity 10.7 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:

Copper ions carry a positive charge and would be attracted towards the negative terminal. Since we want to transfer copper from the thick impure copper rod to the thin copper strip, the thick copper rod should be connected to the positive terminal of the battery and consequently the thin copper strip to the negative terminal of the battery.