NCERT Solutions for Class 8th Science Chapter 8 Chemical Effects of Electric Current based on the latest syllabus of CBSE and JKBOSE NCERT Books
Exercises
1. Fill in the blanks:
(a) Most liquids that conduct electricity are solutions of ____________, ____________ and ____________.
- Acids, bases, salts
(b) The passage of an electric current through a solution causes ____________ effects.
- Chemical
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the ____________ terminal of the battery.
- Negative
(d) The process of depositing a layer of any desired metal on another material by means of electricity is called ____________.
- Electroplating
2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?
- The deflection of the magnetic needle indicates that the solution conducts electricity, allowing current to pass through and create a magnetic field that affects the needle.
3. Name three liquids, which when tested in the manner shown in Fig. 8.9, may cause the magnetic needle to deflect.
- Lemon juice, saltwater, vinegar
4. The bulb does not glow in the setup shown in Fig. 8.10. List the possible reasons. Explain your answer.
- The liquid may not be a good conductor of electricity.
- The connections may be loose.
- The bulb may be fused.
- The battery may be dead.
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.
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.
- (i) Liquid A is a better conductor than liquid B.
6. Does pure water conduct electricity? If not, what can we do to make it conducting?
- Pure water does not conduct electricity. Adding a small amount of salt or an acid or base can make it conducting.
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.
- Water is a good conductor of electricity. Shutting off the electrical supply prevents the risk of electrocution while using water to extinguish the fire.
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?
- Seawater contains a higher concentration of salts and minerals, making it a better conductor of electricity than drinking water.
9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
- No, it is not safe. Water conducts electricity, and there is a risk of electrocution. Additionally, handling electrical equipment in wet conditions can lead to accidents.
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?
- Rainwater contains dissolved salts and minerals from the atmosphere, making it a conductor of electricity.
11. Prepare a list of objects around you that are electroplated.
- Jewelry, cutlery, car parts, bathroom fittings, electronic components
12. The process that you saw in Activity 8.7 is used for the 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?
- The impure copper rod should be attached to the positive terminal because during electrolysis, copper ions from the impure rod move to the negative terminal, where pure copper is deposited.
Extended Learning — Activities and Projects
- Test the conduction of electricity through various fruits and vegetables. Display your result in a tabular form.
- Conduct experiments with different fruits and vegetables.
- Record whether they conduct electricity and present the results in a table.
- Repeat the Activity 8.7 with a zinc plate in place of the copper plate connected to the negative terminal of the battery. Now replace the zinc plate with some other metallic object and again repeat the activity. Which metal gets deposited over which other metal? Discuss your findings with your friends.
- Perform the electroplating activity with different metals.
- Observe and discuss the deposition of metals.
- Find out if there is a commercial electroplating unit in your town. What objects are electroplated there and for what purpose? (The process of electroplating in a commercial unit is much more complex than what we did in Activity 8.7). Find out how they dispose of the chemicals they discard.
- Visit a commercial electroplating unit.
- Learn about the objects they electroplate and their disposal methods for chemicals.
- Imagine that you are an ‘entrepreneur’ and have been provided a loan by a bank to set up a small electroplating unit. What object would you like to electroplate and for what purpose? (Look up the meaning of ‘entrepreneur’ in a dictionary).
- Choose an object to electroplate, such as jewelry or car parts.
- Explain the purpose and benefits of electroplating that object.
