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

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Updated at 2021-05-07


NCERT solutions for class 8 Science Chapter 14 Chemical Effects Of Electric Current Exercise

Q.1 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 _______



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


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.

EXPLANATION

Acids produce the hydrogen ion (H+) while bases produce the hydroxide ion (OH-) when dissolved in water. These ions in water act as charge carriers and can hence help the acids and bases to conduct electricity. Salts produce both cations and anions when dissolved in water, which are conducting particles. Hence salts conduct electricity.

The passage of an electric current through a conducting solution causes chemical effects. That may cause formation of bubbles of a gas on the electrodes, deposition of metal on electrodes, changes of colour of solutions etc.

As copper ions are positive ions, hence they get deposited on the plate connected to the negative terminal of the battery.

Electroplating is the process of applying a metal coating on another piece of metal or on another conductive surface through an electro-deposition process.

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



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


Answer :

The deflection in the compass needle shows that the current is flowing through the wire and hence the current is flowing through the circuit.

Now when the free ends of a tester are dipped into a solution, the magnetic needle shows deflection.

That means the solution is certainly a conducting solution. And all the materials used in this case are conducting materials and thus the circuit becomes complete and the needle shows deflection.

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



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


Answer :

Liquids like lemon juice, salt water and vinegar will allow the electricity to pass through them.

Lemon juice has citric acid in it. When lemon juice is added to water, the acid dissociates into anions and cations which are charged particles or ions. As these charged particles are able to flow inside the acid, hence they can conduct electricity.

Salt water has sodium ions and chlorine ions which are conducting particles. Hence salt water can conduct electricity.

Vinegar has acetic acid in it. When vinegar is added to water, the acid dissociates into anions and cations which are charged particles or ions. As these charged particles are able to flow inside the acid, hence they can conduct electricity.

Q.4 The bulb does not glow in the setup shown in given Fig List the possible reasons. Explain your answer.



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


Answer :

The bulb may not glow because of the following reasons:

(i) The battery may be used up or dead.

(ii) The liquid may be an insulator which is a poor conductor of electricity.

(iii) The bulb used may be a fused one.

Q.5 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.



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


Answer :

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. We would conclude that (i) liquid A is a better conductor than liquid B.

EXPLANATION

When the current flows through the tester then the neon bulb glows as it takes very little current to light. When the current doesn't flow through the tester, the circuit of the tester is not complete and the bulb does not glow. In case of liquid A, the bulb in the tester glows, which means liquid A is a good conductor while the same is not seen in case of liquid B.

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



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


Answer :

No, pure water does not conduct electricity.

Because pure water lacks different minerals and ions which can conduct electricity.

Now if we add some salt in the pure water then water will start conducting electricity.

Q.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.



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


Answer :

In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. This is because the water contains various minerals and ions which are conducting particles.

As water may conduct electricity and in case of leakage if the water is poured over the working electrical appliances then the electric current may pass through the water to everyone in contact with water and thus can harm the firemen.

Q.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 ex¬plain the reason?



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


Answer :

The seawater contains a large amount of salts and ions in comparison to drinking water.

These salts and ions can easily conduct electricity and thus seawater is a better conductor of electricity and produces a stronger magnetic field in the wire.

Hence when he tested both seawater and drinking water with his tester then the compass needle shows more deflection in the case of seawater.

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



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


Answer :

No, it is not safe to repair electrical appliances outdoors during heavy downpour.

This is because rain water contains dissolved salts and ions which are conducting particles.

As the rain water can conduct electricity so the electrician may get electrical shocks while working outdoors during heavy downpour.

Q.10 Paheli had heard that rain water is as good as distilled water. So, she collected some rain water 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?



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


Answer :

Rain water is the pure water which lacks impurities and dust particles. At this stage it can called as distilled water as it lacks conducting particles and ions.

The rain water behaves as an insulator until unless it gets mixed with air pollutants like sulphur dioxide, NOx (different oxides of nitrogen) and dust particles which makes the rainwater acidic as well as a good conductor of electricity.

So, when Paheli tested the rainwater using a tester, the compass needle showed deflection..

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



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


Answer :

Examples of electroplated objects around us are as follows:

(i) Gold plating :
A fine layer of gold is deposited on the silver ornaments and thus they are called gold-plated ornaments.

(ii) Zinc plating :
Iron used in construction purposes, is coated with a layer of zinc. As zinc coating protects iron from corrosion and rusting due to the presence of moisture in atmosphere.

(iii) Chromium plating :
Chromium plating is applied on the exterior parts of automobiles in order to obtain a shiny appearance.

Q.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 the battery and why?



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


Answer :

Copper ions are positively charged ions. So the copper ions will be attracted towards the plate which is connected to the negative terminal of the battery.

As copper ions from the thick impure copper rode must be transferred to the thin pure copper plate, so the thin pure copper plate must be connected to the negative terminal of the battery.

Consequently, the impure copper rod will be connected to the positive terminal of the battery so that the positive copper ions from the impure copper rod will transfer to the negative terminal of the thin pure copper plate.



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