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SEBA Class 8 Science Chapter 8 Chemical Effects Of Electric Current
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Chemical Effects Of Electric Current
Chapter: 8
PART – I
EXERCISE
1. Fill in the blanks.
(a) Most liquids that conduct electricity are solutions of __________ , ___________ and ____________.
Ans: Most liquids that conduct electricity are solutions of acids, bases, and salts.
(b) The passage of an electric current through a solution causes _____________ effects.
Ans: 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 ___________ terminal of the battery.
Ans: 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 __________.
Ans: 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?
Ans: When the two free ends of a tester are dipped into a solution, if the solution can conduct electricity, an electric current starts flowing through the tester. This flow of electricity creates a magnetic field inside the tester’s coil, which causes the magnetic needle to move (deflect). The deflection of the needle indicates that electric current is passing through the solution, meaning the solution is a conductor of electricity. If the solution does not conduct electricity, no current flows, and the needle does not move. Therefore, the movement of the needle shows that the solution contains charged particles (ions) that help carry the electric current, allowing the tester to detect whether the solution is conductive or not.
3. Name three liquids, which when tested in the manner shown in Fig. 8.9. may cause the magnetic needle to deflect.
Ans: Here are three more liquids that can cause the magnetic needle to deflect in the same setup:
(i) Lemon juice: contains citric acid, which ionizes and conducts electricity.
(ii) Tap water: contains dissolved minerals and impurities that make it conductive.
(iii) Copper sulfate solution: a salt solution that conducts electricity due to copper and sulfate ions.
4. The bulb does not glow in the setup shown in Fig. 8.10. List the possible reasons, Explain your answer.
Ans: For the bulb to glow, there must be a closed circuit with a continuous path for the electric current to flow from the battery through the bulb and back. In the setup, the liquid in the container acts as a part of the circuit. If the liquid does not conduct electricity, the circuit is open at that point, and the bulb will not glow. Similarly, any fault in wiring, bulb, or battery will also prevent current flow, causing the bulb to remain off.
The bulb does not glow because the circuit is incomplete. Possible reasons are:
(i) The liquid in the container does not conduct electricity.
(ii) There is a loose or broken connection in the circuit.
(iii) The bulb is faulty.
(iv) The battery is dead.
5. A tester is used to check the conduction of electricity through two liquids, labelled ‘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.
Ans: (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?
Ans: Pure water does not conduct electricity because it does not have free electrons or ions to conduct electricity. Pure water can conduct electricity when a pinch of common salt is added to it, as salt solution is conducting in nature.
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.
Ans: Firemen shut off the main electrical supply before using water hoses because water conducts electricity. If the electricity is not turned off, water can carry electric current, which may cause electric shocks to the firemen and worsen the danger. Turning off the electricity makes it safer to use 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?
Ans: The compass needle deflects more in seawater because seawater contains a high amount of dissolved salts, especially sodium chloride, which makes it a much better conductor of electricity than drinking water. The higher concentration of ions in seawater allows more electric current to flow, causing a stronger deflection of the compass needle compared to the relatively purer drinking water.
9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Ans: No, it is not safe for an electrician to carry out electrical repairs outdoors during heavy downpour. Water is a good conductor of electricity, and working with electrical equipment in wet conditions increases the risk of electric shock or electrocution. The wet environment can cause short circuits and accidents. For safety, electrical repairs should be done only when the area is dry and power is switched off.
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?
Ans: Paheli thought rainwater was as pure as distilled water, but when she tested it with a tester, the compass needle showed deflection. This happened because rainwater is not completely pure like distilled water. It contains dissolved gases such as carbon dioxide from the air, as well as other impurities and dust particles picked up while falling through the atmosphere. These dissolved substances make rainwater slightly conductive, allowing electric current to pass through it. That’s why the compass needle deflected during the test.
11. Prepare a list of objects around you that are electroplated.
Ans: There are many objects around us that are electroplated. Electroplating is a process in which a thin layer of metal is coated on the surface of an object to make it shiny, strong, and resistant to rust.
Here’s a list of common objects around us that are often electroplated:
(i) Jewelry (like rings, bracelets, necklaces): Often plated with gold, silver, or rhodium.
(ii) Cutlery and utensils: Plated with silver or stainless steel coating.
(iii) Watches: Many have gold or silver plating for shine and durability.
(iv) Coins: Sometimes plated with metals like nickel or copper.
(v) Car parts: Such as bumpers and rims, often chrome-plated for corrosion resistance and shine.
(vi) Electrical connectors and pins: Usually plated with gold or silver to improve conductivity.
(vii) Bathroom fittings: Taps, showerheads, and handles plated with chrome or nickel for rust resistance.
(viii) Eyeglass frames: Some are electroplated for aesthetics and protection.
(ix) Musical instruments: Brass or silver plating on parts for protection and looks.
(x) Mobile phone cases and accessories: Sometimes plated with decorative metals.
12. The process that you saw in Activity 8.7 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 the battery and why?
Ans: The impure copper rod (anode) connected to the positive terminal loses copper atoms, which dissolve into the solution as copper ions (Cu²⁺). These copper ions then move through the solution and deposit onto the pure copper plate (cathode) connected to the negative terminal. Thus, copper is transferred from the impure anode to the pure cathode, resulting in the purification of copper.
So, the thick impure copper rod must be connected to the positive terminal because it acts as the anode from where copper dissolves, and the thin pure copper plate is connected to the negative terminal to collect the pure copper deposit.
This is the standard electrorefining process of copper purification where the positive terminal is connected to the impure copper electrode to dissolve impure copper into ions, which then get deposited on the cathode (negative terminal) as pure copper.
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