Monday, 8 April 2013

Chapter 14. CHEMICAL EFFECTS OF ELECTRIC CURRENT | Science | CBSE Class VIII (8th) | Solved Exercises

Things to remember....

  1. Some liquids are good conductors of electricity and some are poor conductors.
  2. Most liquids that conduct electricity are solutions of acids, bases and salts.
  3. The passage of an electric current through a conducting liquid causes chemical reactions.
  4. The resulting effects are called chemical effects of currents.
  5. The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating.
Question 1. Fill in the blanks
  1. (a) Most liquids that conduct electricity are solutions of ,______________ and ______________.
  2. (b) The passage of an electric current through a solution causes ______________ effects.
  3. (c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the __________ terminal of the battery.
  4. (d) The process of depositing a layer of any desired metal on another material by means of electricity is called _________ .
Answer :
  1. (a) Most liquids that conduct electricity are solutions of , acids, bases and salts.
  2. (b) The passage of an electric current through a solution causes chemical reactions effects.
  3. (c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the Negative terminal of the battery.
  4. (d) The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating .

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

Answer : When the free ends of a tester are dipped into a solution, and the magnetic needle shows deflection, It implies that the solution is good conductor of electricity and it is allowing electric current to pass through its passage between the dipped free ends of tester thus making the circuit complete. The electric current passing through conducting wire of tester, due to magnetic effect of electric current, produces magnetic field around it, which causes the deflection of magnetic needle.




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

Answer :
  1. Lemon juice
  2. Vinegar
  3. Common Salt Solution


Question 4. The bulb does not glow in the setup shown in Figure. List the possible reasons. Explain your answer.



Answer :
  1. The Liquid is either bad or poor conductor of Electricity
  2. The connections are loose
  3. The bulb is fused
  4. The cells being old or used up, are not producing electric current


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. (i) liquid A is a better conductor than liquid B.
  2. (ii) liquid B is a better conductor than liquid A.
  3. (iii) both liquids are equally conducting.
  4. (iv) conducting properties of liquid cannot be compared in this manner.

Answer : (i) 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 : The pure water does not conduct electricity as it is poor conductor of electricity. We can make pure water conducting by adding either some common salt, or few drops of acidic solution such as lemon juice or dilute hydrochloric acid

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 : In case of fire, the firemen shut off the main electrical supply for the area, before they use the water hoses for the following reasons :

  1. (i) The melting of electric wire insulation, resulting from the heat, may cause short circuit. It may result in escalating the fire in the area
  2. (ii)The using of water for putting off the fire will be dangerous, as melting of electric wire insulation, together with increased conductivity of wet surface, will lead to the leakage of electric current in the area, resulting in electric shocks to struggling inhabitants and fire men rescuing them.
Question 8. A child staying in a 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 : We know that pure water is a poor conductor of electric current. The presence of some impurities in the form of some salt or acid, turns it to a good conductor. The seawater and the drinking water both contains salt impurities dissolved in them. The salt concentration in seawater is much more as compare to the normal drinking water. The presence of excess of salt in seawater makes it as better conductor of electricity than drinking water. Thus the child testing the the drinking water and the seawater, finds compass needle deflecting more in the case of seawater, due to excess flow of current in the tester.

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

Answer : The rain water, mixed with chemical impurities such as acids or salts in the air and ground surface, becomes good conductor of electricity. Therefor it would not be safe for the electrician to carry out electrical repairs outdoors during heavy downpour, as risk of electric shock due to leakage of current, working on a wet surface, will be always there.

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 reasons?

Answer : The rainwater is supposed to be as good as distilled water. But now a days due to increased level of air pollution, a lot of impurities or pollutants are present in the air.These impurities may be acidic or may contain some salt. All these impurities in the form of either acid or salt, get dissolved with the rainwater, and make it good conductor of electricity. This is for the reason, why paheli found the compass needle showing deflection on testing with a tester.

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

Answer : The list of electroplated objects around us
  1. (i) The Gold Plated Artificial Jewellery
  2. (ii) The chromium plated Automobiles parts such as Wheel Rim, Engine Parts and Carriage etc
  3. (iii) The chromium  plated kitchen and bathroom fixtures, such as Tapes, Showers, Water Sinks, Racks.
  4. (iv) Tin platted iron cans, used for storing food,
  5. (v) The copper plated metal decorations for home and offices
  6. (vi) The shaving blade with titanium coating on  cutting age.
  7. (vii) The chrome plated Surgical Instrument

Question 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 battery and why?


Answer : The electrode with thick rod of impure copper should be attached to the positive terminal of battery because in electroplating process, When electric current is passed through the copper sulphate solution,copper sulphate dissociates into copper and sulphate. The free copper gets drawn to the electrode connected to the negative terminal of the battery and gets deposited on it. An equal amount of copper gets dissolved from the other electrode with thick rod of impure copper, in the solution. Thus,the loss of copper from the solution is restored .As the process keeps going , the copper gets transferred from positive electrode with thick rod of impure copper to the other electrode made up of thin plate of pure copper..

Did You Know?

LEDs (Light Emitting Diodes) are available in many colours such as red, green, yellow, blue, white and are increasingly being used for many applications, for example in traffic signal lights. LEDs are increasingly being used for lighting. A cluster of white LEDs grouped together forms a LED light source. LED light sources consume less electricity and have longer lifetime than light bulbs and fluorescent tubes. But LED light sources are expensive, so CFLs are currently the best choice. However, CFLs contain mercury 




which is toxic. Therefore, used or broken CFLs need to be disposed off safely. Once the technological advances reduce the cost of LEDs, they will become the preferred lighting source.

CBSE Class VIII (8th)  |  Science |  | Chapter 14  CHEMICAL EFFECTS OF ELECTRIC CURRENT | Solved Exercises

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