SCIENCE CLASS 8 CHAPTER 14 CHEMICAL EFFECT OF ELECTRIC CURRENT
NCERT Class 8 Science – Chapter 14
Chemical Effects of Electric Current
A. Textbook exercise questions and answers
Q1. Fill in the blanks.
(a) Most liquids that conduct electricity are solutions of ______, ______ and ______.
Answer: Acids, bases and salts.
(b) The passage of an electric current through a solution causes ______ effects.
Answer: Chemical effects.
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the ______ terminal of the battery.
Answer: Negative terminal.
(d) The process of depositing a layer of any desired metal on another material by means of electricity is called ______.
Answer: 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 magnetic needle deflects because electric current flows through the circuit. This indicates that the solution conducts electricity. The current produces a magnetic effect around the wire.
Q3. Name three liquids which may cause the magnetic needle to deflect when tested.
Answer:
Lemon juice
Salt solution
Vinegar
These liquids contain ions that allow electric current to pass through them.
Q4. The bulb does not glow in a tester. List the possible reasons.
Answer: The bulb may not glow because:
The bulb is fused.
The cells are exhausted.
The circuit connections are loose.
The liquid does not conduct electricity.
The current flowing through the liquid is too weak to make the bulb glow.
Q5. A tester is used to check two liquids, A and B. The bulb glows brightly in liquid A but very dimly in liquid B. What can you conclude?
Answer: Liquid A is a better conductor of electricity than liquid B, assuming the same tester and conditions are used.
Q6. Does pure water conduct electricity? If not, what can we do to make it conducting?
Answer: Pure water is a very poor conductor of electricity because it contains very few ions. We can make it conduct electricity by dissolving a small amount of salt in it. Never perform this experiment using household mains electricity.
Q7. Why is it dangerous to touch electrical appliances with wet hands?
Answer: Water containing dissolved salts conducts electricity. Wet hands can provide a path for electric current through the body, causing an electric shock. Therefore, electrical appliances should never be touched with wet hands.
Q8. Why is it recommended that the electrician wear rubber gloves while repairing electrical appliances?
Answer: Rubber is an electrical insulator. Rubber gloves can help reduce the risk of electric shock when they are properly rated and used. However, gloves alone do not make electrical work safe; the power must be switched off and the circuit isolated.
Q9. Is it safe for an electrician to carry out electrical repairs outdoors during heavy rain? Explain.
Answer: No, it is not safe. Rainwater and wet surfaces can conduct electricity because they contain dissolved substances. This increases the risk of electric shock. Electrical repairs should be postponed until conditions are safe and the power supply has been isolated.
Q10. Paheli collected rainwater in a clean glass tumbler and tested it. The magnetic needle showed deflection. What could be the reason?
Answer: Rainwater dissolves gases and picks up dust and other substances from the atmosphere. These dissolved substances can provide ions that allow electric current to flow. Therefore, the magnetic needle may deflect.
Q11. Prepare a list of objects around you that are electroplated.
Answer: Examples of electroplated objects include:
Chrome-plated taps and bathroom fittings.
Gold-plated jewellery.
Silver-plated ornaments and utensils.
Tin-plated food cans.
Chrome-plated bicycle and motorcycle parts.
Nickel-plated door handles.
Q12. Imagine you are an electroplating factory owner. What steps would you take to prevent pollution caused by waste materials?
Answer: I would treat the wastewater before releasing it, recover and reuse valuable metals where possible, dispose of hazardous chemicals according to environmental regulations, and regularly monitor the quality of wastewater. This would help prevent soil and water pollution.
B. Important additional questions
Q13. What are the chemical effects of electric current?
Answer: When electric current passes through a conducting liquid, it may cause chemical changes such as:
Formation of gas bubbles at electrodes.
Deposition of metal on an electrode.
Change in the colour of the solution.
Q14. What is electroplating?
Answer: Electroplating is the process of depositing a thin layer of one metal over another material using electric current.
Q15. Write three advantages of electroplating.
Answer:
It protects metals from corrosion.
It improves the appearance of objects.
It can make objects more resistant to wear.
Q16. Why is chromium used for electroplating?
Answer: Chromium is shiny, resists corrosion and is hard. It is used to coat objects such as taps, bicycle parts and vehicle components.
Q17. What are electrodes?
Answer: Electrodes are conducting rods or plates through which electric current enters or leaves a liquid or solution.
Q18. Why might an LED glow when an ordinary bulb does not glow in a tester?
Answer: An LED can glow even when the current is very small. An ordinary bulb may require more current to heat its filament enough to glow.
Q19. What is the role of ions in conducting liquids?
Answer: Ions are electrically charged particles. When a voltage is applied, positive and negative ions move in opposite directions and help carry electric current through a conducting liquid.
Q20. Why should electroplating waste not be released directly into rivers?
Answer: Electroplating waste may contain toxic metals and harmful chemicals. Releasing it directly into rivers can pollute water, harm aquatic organisms and threaten human health.
C. Quick revision notes
Term | Meaning |
|---|---|
Conductor | A material that allows electric current to pass through it |
Insulator | A material that resists the flow of electric current |
Chemical effect | Chemical change caused by electric current |
Electrode | A conductor through which current enters or leaves a solution |
Electroplating | Coating a material with a thin metal layer using electricity |
LED | Light-emitting diode; can glow with a small current |
Important exam points to remember:
Solutions of acids, bases and salts can conduct electricity.
Pure or distilled water is a very poor conductor.
Electric current can produce chemical, heating and magnetic effects.
Copper from copper sulphate solution is deposited on the electrode connected to the negative terminal under the usual electroplating setup.
Electroplating helps protect metals and improve their appearance.
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