Monday, November 26, 2012

CBSE Class 8 - Science - CH 14 - CHEMICAL EFFECTS OF ELECTRIC CURRENT

CHEMICAL EFFECTS OF ELECTRIC CURRENT

CHEMICAL EFFECTS OF ELECTRIC CURRENT
NCERT Solutions, Q & A

Q1. Fill in the blanks

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 deflection of magnetic needle indicates electric current is flowing through the wire. It means the liquid or the solution is a good conductor of electricity. When the free ends of a tester are dipped into a solution, electric circuit is completed and an electric current passes through the solution.

Q3. Name three liquids, which when tested in the manner shown in Figure 14.9, may cause the magnetic needle to deflect.

Answer: Tap water, vinegar, lime juice. The liquid solutions containing salts (basic or acidic) will conduct electricity.

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


Answer: Following may be the possible reasons the bulb does not glow:
  1. Liquid solution in container is a poor conductor of electricity. For example distilled water.
  2. Bulb may be fused.
  3. The battery has already been exhausted, there are no sufficient charges (electric current) required for the bulb to glow.
  4. Any loose connection (air gaps) like electrodes are not properly connected to wire.
Q5. 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
(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.

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

Answer: Pure or distilled water does not contain salts. Therefore it is a poor conductor of electricity. We can add (e.g. table salt) to make it conducting.

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: The water usually contain salts and is a good conductor of electricity. To save themselves and others from electrocution and to avoid any short circuit, firemen shut off the main electrical supply for the area.

Q8. 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: Drinking water before supply, is chemically treated and various impurities and salts are removed from it. While the sea water contains lots of mineral salts. Therefore, sea water produce more ions (more electric charges) as compared to drinking water and the child sees more deflection in magnetic needle in case of sea water.

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

Answer: No it is very risky and unsafe to carry out electric repairs outdoors during heavy downpour. Water is a good conductor of electricity and there are high chances of electrocution.

Q 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: Air in our surroundings contains salts of dust particles, pollutants, suspended particles. These salts gets dissolved in rain water and make it good conducting medium of electricity.

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

Answer: Car bumpers and cycle handles are chromium plated. Artificial Jewellery items are silver or gold-plated. Almirah, door handles are zinc plated. Cold drink cans are tin plated.

Q12. 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: Impure Copper plate should be connected to positive terminal. Pure copper plate should be connected to negative terminal of electrode. Copper ions are positively charged and will attract to negative electrode terminal.

Q13: What do you mean by electroplating?

Answer: The process of depositing a layer of any desired metal on another material, by means of
electricity, is called electroplating.

Q14(MCQ): Iron is galvanized by coating with__________________.
 

(a) nickel 
(b) tin 
(c) zinc 
(d) copper

Answer: (c) zinc.
Galvanization is a process of electroplating zinc coating to iron or steel to protect it from rusting.


Q15: What is LED?

Answer: LED stands for Light Emitting Diodes. LED light sources consume less electricity and have longer lifetime than light bulbs and fluorescent tubes. These are used in traffic lights, TV screens etc.


Q16: Name the scientist who showed electrolysis of water.

Answer: William Nicholson


Q17: Why are Tin cans used for storing food?

Answer: Tin cans are made by electroplating tin onto iron. Tin is less reactive than iron. Thus, food
does not come into contact with iron and is protected from getting spoilt. That's why tin cans are used for storing food.

Q18: Which type of energy is converted into electrical energy in an electric cell?

Answer: Chemical energy.


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