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

NCERT
SOLUTIONS
CLASS - 8TH

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Book : Science Ncert Solutions | Chapter-14 Science

Class : 8th
Subject : Science
Chapter : 14
Chapter Name : 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.
(The solutions of acids, bases or salts are conducting in nature. They allow the current to pass
through.)
(b) The passage of an electric current through a solution causes chemical effects.
(When an electric current passes through a solution, the solution decomposes into its positive and
negative ions. This process of decomposition of the solution is a chemical effect.)
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate
connected to the negative terminal of the battery.
(When an electric current passes through a copper sulphate solution, the solution decomposes
into positively charged copper ions and negatively charged sulphate ions. These positively charged
copper ions get attracted towards the plate which is connected to the negative terminal of a
battery.)
(d) The process of depositing a layer of any desired metal on another material by means of
electricity is called electroplating.

Page : 180 , Block Name : Exercise

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

Answer. The de ection in the compass needle shows that current is owing through the wounded
wire and hence, through the circuit. The circuit is complete since free ends of the tester are dipped
in a solution. The solution is certainly a conducting solution. This is the reason why the compass
needle shows a de ection.

Page : 180 , Block Name : Exercise

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Book : Science Ncert Solutions | Chapter-14 Science

Q3 Name three liquids, which when tested in the manner shown in Fig.14.9, may cause the
magnetic needle to de ect.

Answer. Liquids like lemon juice, salt water and vegetable oil allow electricity to pass through
them. Hence, these liquids can be used as in the beaker to show the given effect.

Page : 180 , Block Name : Exercise

Q4 The bulb does not glow in the setup shown in Fig.14.10. List the possible reasons. Explain your
answer.

Answer. The bulb may not glow because of the following reasons:
(i) Liquid in the beaker is non-conducting. In such case, the electric current would not be able to
pass through the liquid. Hence, the circuit is not complete.
(ii) Electric current in the circuit is very weak. This can happen if the material used for making the
circuit is not a good conductor of electricity or the battery does not have suf cient energy to
generate electricity.

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Book : Science Ncert Solutions | Chapter-14 Science

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 amount of current owing through a conducting solution depends on the conductivity of the
solution. With more conductivity, more current passes through the solution and vice-versa. Hence,
the conductivity of liquid A is more than the conductivity of 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. This is because pure water is devoid of any
salts. Pure water can conduct electricity when a pinch of common salt is added to it, as salt
solution is conducting in nature.

Q7 In case of a re, before the remen use the water hoses, they shut off the main electrical supply
for the area. Explain why they do this.

Answer. Water may conduct electricity. If the electrical supply for the area is not shut off and water
is poured over electrical appliances, then electricity may pass through water and harm the remen.
That is why, in case of a re, the remen shut off the main electrical supply for the area before
they use the water hoses.

Q8 A child staying in a coastal region tests the drinking water and also the seawater with his
tester. He nds that the compass needle de ects more in the case of seawater. Can you explain the
reason?

Answer. Sea water contains more dissolved salts than the drinking water. Hence, it is more
conducting than the drinking water. Because of this reason, the compass needle de ects more in
seawater than in the drinking water.

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

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Book : Science Ncert Solutions | Chapter-14 Science

Answer. No. It is not safe to repair electrical appliances outdoors during heavy downpour. This is
because rain water contains dissolved salts. Therefore, rain water can conduct electricity. The
electrician may get electrical shocks while working outdoors during rain.

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 show de ection. What could be the reasons?

Answer.Rain water contains dissolved salts. This makes it a conducting solution. There are no
dissolved salts present in the distilled water. Hence, rain water can allow electricity to pass
through it while distilled water cannot.

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

Answer. Prepare a list of objects around you that are electroplated.
Examples of electroplated objects are as follows:
(i) Chromium plating is done on different parts of cars, buses and motorcycles to give them shiny
appearance.
(ii) A ne layer of gold is deposited on the silver ornaments and they are called gold-plated
ornaments.
(iii) Iron used in constructing a building is coated with a layer of zinc. This protects iron from
corrosion and rusting.

Q12 The process that you saw in Activity 14.7 is used for puri cation 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 ion is positively charged. It is 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 must be connected to the negative terminal of the battery. Consequently,
impure copper rod is connected to the positive terminal of the battery.

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