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Page No 179:

Question 1:

In which of the following will the bulb light up?
Figure

(a) all the liquids will conduct electricity
(b) only B and D
(c) B, C and D
(d) A, B, C and E

Answer:

(b) Only B and D

These liquids contain dissolved salts, which produce ions to conduct electricity.

Page No 179:

Question 2:

The diagram given here shows a beaker containing a solution of copper sulphate and two carbon electrodes. A battery is placed next to it. In order that the electrode E be plated with copper
(a) E must be connected to H and F to G.
(b) E must be connected to F and H to G.
(c) E must be connected to G and F to H.
(d) E and F must be connected to G.
Figure

Answer:

 (a) E must be connected to H and F to G.

In order that electrode E is plated with copper, E must be connected to H, and F to G.

Page No 179:

Question 3:

In the above experiment, if E were plated with copper, which would be the positive electrode?
(a) E
(b) F
(c) G
(d) H

Answer:

(b) F

F would be the positive electrode in the experiment.

Page No 179:

Question 4:

The bulb in the circuit does not glow. What could be the reason for this?
(a) The liquid in the beaker is sugar solution.
(b) The bulb is fused.
(c) The insulation from the ends of the wires was not removed.
(d) all the above
Figure

Answer:

(d) All the above

In the above figure, the bulb in the circuit does not glow if
(a) the liquid in the beaker is a sugar solution
(b) the bulb is fused
(c) the insulation from the ends of the wires is not removed

Page No 179:

Question 5:

Which of the following methods can make pure water conduct electricity?
A. dissolve some sugar in it
B. dissolve some salt in it
C. add hydrochloric acid to it
D. heat the water sample

(a) A and D
(b) B and C
(c) A, B and C
(d) only B

Answer:

(b) B and C

By dissolving some salt in pure water or by adding hydrochloric acid to it, it can be made to conduct electricity.



Page No 180:

Question 6:

Which one of the following is not an electrolyte?
(a) salt solution
(b) sulphuric acid in water
(c) copper sulphate solution
(d) mercury

Answer:

(d) Mercury

Mercury is not an electrolyte.

Page No 180:

Question 7:

The passage of electric current through an electrolyte causes
(a) chemical changes.
(b) rise in temperature.
(c) physical changes.
(d) evolution of gas in all cases.

Answer:

(a) chemical changes

The passage of electric current through an electrolyte causes chemical changes.

Page No 180:

Question 8:

The breakdown of a chemical compound using electricity is known as
(a) distillation.
(b) electroplating.
(c) electroplating.
(d) conduction.

Answer:

(b) electrolysis

The breaking down of a chemical compound using electricity is known as electrolysis.

Page No 180:

Question 9:

Explain the term electrolysis.

Answer:

Electrolysis:
Pure water is a very poor conductor of electricity. When some salts are dissolved in it, a chemical compound that conducts electricity, i.e., an electrolyte, is formed. Thus, the breaking down of a chemical compound using electricity is called electrolysis.

Page No 180:

Question 10:

What is an electrolyte? Name two electrolytes and two non-electrolytes.

Answer:

Electrolytes are compounds that conduct electricity when they are in a solution or in a molten state.

Two examples of electrolytes are NaCl (sodium chloride) and CuSO4 (copper sulphate)

.

Two examples of non-electrolytes are distilled water and kerosene.

Page No 180:

Question 11:

Four substances were tested for their electrical conductivity. The results are shown in the table below. Study the table and answer the questions that follow.

Substance Bulb Substance at Negative
Electrode
Substance at Positive
Electrode
A lights up hydrogen chlorine
B does not light nothing nothing
C lights up nothing nothing
D lights up copper oxygen

(a) Which substances conduct electricity?
(b) Which one is a non-electrolyte?
(c) Which one could be mercury?
(d) Which solution could be:
 i. sodium chloride
ii. copper sulphate
iii. kerosene

Answer:

(a) The substances that conduct electricity are A , C and D.

(b) B and C are non-electrolytes.

(c) C could be mercury.

(d) The solution that could be
 i. sodium chloride - A
ii. copper sulphate - D
iii. kerosene - B

Page No 180:

Question 12:

Rajesh bought a bottle that was labelled as distilled water. He suspected it to be tap water. Draw and explain what test he could use to find out for sure?

Answer:

Diagram:

Rajesh completed the circuit as shown in the diagram and watched the electrodes carefully. He noticed bubbles collecting at the electrodes, which indicated the formation of gases. Tap water contains several dissolved salts that allow electricity to flow through it, and in this process, tap water break downs into hydrogen and oxygen gases. Distilled water does not contain any dissolved salts, so it is a poor conductor of electricity. Therefore, no gases are formed at electrodes placed in it. 
From this test, Rajesh confirmed that the water in the bottle was tap water and not distilled water.

Page No 180:

Question 13:

Define electroplating. List two uses of this process. Draw a labelled diagram to show how you would electroplate an iron nail with copper.

Answer:

The process of coating one metal over another with the help of electricity is called electroplating.
The following are the uses of electroplating.
  1. To make objects more attractive
  2. To prevent an object from rusting
Electroplating an iron nail with copper:

 

For electroplating an iron nail with copper, we need a copper sulphate solution, a beaker, an iron nail, a copper wire and a battery.
We will make the connections as shown in the figure and wait for some time. On passage of an electric current, the positively charged copper ions from the solution go to the negatively charged nail and get deposited there. After the process of electroplating is over, we will notice a reddish-brown deposit on the iron nail, which is copper. The nail gets coated with copper.
 

Page No 180:

Question 14:

Neither dry soil nor pure water conducts electricity. Yet wet soil will conduct electricity. Explain why this happens.

Answer:

Pure water contains no salts. Soil contains several salts that can produce ions (charged particles). When soil is dissolved in water, the salts in the soil break down into ions, which freely move in the solution to conduct electricity. Thus, soil conducts electricity only when it is wet.

Page No 180:

Question 15:

Mercury is a conductor but not an electrolyte. Tap water is a conductor and an electrolyte. Explain these two statements.

Answer:

Electrolytes let electricity pass through them and, at the same time, are chemically changed by it.
Mercury remains unchanged when it conducts electricity. Hence, it is not an electrolyte.



Page No 181:

Question 16:

Vimal made a circuit as shown in the figure. He noticed that the magnetic needle showed deflection. Can you explain his observation?
Figure

Answer:

The solution shown in the figure is a good conductor of electricity. There is a current in the outer circuit, and therefore, when a magnetic needle is brought near the circuit, the needle gets deflected owing to the magnetic effect of the electric current.

Page No 181:

Question 17:

List two uses of electrolysis.

Answer:

The following are two uses of electrolysis.
  1. To coat one metal with another by the process of electrolysis.
  2. To purify metals. Also, metals such as sodium and aluminium are obtained from their ores by the process of electrolysis.
   



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