26 November 2011

8th physics Chemical effects of electric current Solved Questions

Physics Questions with their Answers
Q 1. Which of the following liquids conduct electricity and which do not conduct electricity?
Lemon juice, Milk, Vinegar, Salt solution, Distilled water, Honey, Sea water, Rain water.
Ans: Conducting           : Lemon juice, Vinegar, Salt solution, sea water, Rain water
        Non Conducting    : Milk, Distilled water, Honey.
Q 2. What is advantage of using LED over bulb in testing the electrical conductivity of liquids?
Ans: When electric current flows through a bulb then due to heating effect of current,the  filament of the bulb gets heated up to a high temperature ; it starts glowing. Now for a liquids having low electrical conductivity, the current flowing through the circuit is
very weak due to which the filament does not get heated sufficiently & hence the bulb does not glow. Therefore LED is used in place of bulb because LED glows even when weak electric current flows in the circuit.
Q 3. Which effect of electric current is utilized for detecting the flow of current through a solution:   a) When a torch bulb is used?       b) When a magnetic compass is used?
Ans: a) Heating effect      b) Magnetic effect.
Q 4. Distilled water does not conduct electricity. What substances can be added to distilled water in small amounts to make it a good conductor of electricity?
Ans: Salt, acid & base.
Q5. In case of a fire, before the fireman uses the water hoses to throw water to douse fire, they shut off the electricity supply for the area. Explain why this is done?
Ans: To prevent electrocution of fireman because ordinary water is conductor of electricity.
Q6. When the free ends of a conductivity tester (made by using a battery connected to a wire wound around a compass) are dipped into a solution, the magnetic needle shows deflection. Can you give the reason of this deflection?
Ans: Electric current flowing through the wire produces a magnetic field around it. And this magnetic field of electric current acts on the magnetic needle of compass; deflects it.
Q7. What effects does an electric current produce when flowing through a conducting solution?
Ans: When an electric current flows through the conducting solution, it causes a chemical reaction (or chemical change).
These chemical reactions may produce following effects:
i) Bubbles of gas/es may be formed on the electrodes.
ii) Deposits of metals may form on electrodes.
iii) Change in colour of solution may occur.
Q8. When electric current is passed through acidified water then what is produced at
(a) Positive carbon electrode (anode)?          (b) Negative carbon electrode (cathode)?
Ans: a) oxygen gas b) hydrogen gas.
Q9. Which effect of electric current is utilized when a thin layer of chromium metal is deposited on an iron tap? What is this process known as?
Ans: Chemical effect of current is utilized. The process is known as electroplating.
Q10. What is meant by electroplating? What is the purpose of electroplating?
Ans: The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating. Electroplating is done
i) for protection against corrosion (or rusting).
ii) for decorative purposes.
Q11. Which properties of chromium metal make it suitable for electroplating it on car bumpers, bath taps & bicycle handle, etc., made of iron?
Ans It has shiny appearance. It does not corrode easily. It resists scratches.
Q12. Which metal is electroplated on iron for making ‘cans’ used for storing food & Why?
Ans: Tin.  Tin metal has shiny appearance, it does not corrode & it is non poisonous. Tin is less reactive than iron. Due to tin plating over the surface of iron, the food does not come in contact with iron and  is protected from getting spoilt.
Q13. On what factors the chemical effect produced by an electric current depends?
Ans: The chemical effect produced by an electric current depends on the nature of conducting solution (through which it is passed), & on the nature of electrodes used for passing the electric current.
Q14. In the process of purification of copper metal, a thin plate of pure copper and  a thick rod of impure copper are used as electrodes & a metal salt solution is used as an electrolyte:
a) Which electrode is connected to the positive terminal of the battery?
b) Which electrode is connected to the negative terminal of the battery?
c) Which metal salt solution is taken as electrolyte?
Ans: a) Thick rod of impure copper. b) Thin rod of pure copper. c) Copper sulphate solution.
Q15. Write down the important points which should be remembered while electroplating?
 i) The metal on which electroplating is to be done should be cathode i.e., the negative electrode.
ii) The metal to be deposited should be anode i.e., the positive electrode.
iii) A water soluble salt of the ‘metal to be deposited’ is taken as the electrolyte.
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