Case Study Based Questions: Chemical Effects of Electric Current | Science Class 8 PDF Download
Case study 1.
Neha connects two nails to the two terminals of a battery including a torch bulb and a switch in the circuit. She immersed the connection into a solution labelled A in a beaker. When she pass the electric current through the solution A by closing the switch, the bulb starts glowing. Now, she replace the solution A, in the beaker with a solution labelled B. When she switch on the electric current, the bulb does not glow.
Q1: The solution labelled A would be (a) Alcohol solution (b) Hydrochloric acid (c) Distilled water (d) None of these Ans: (b) Hydrochloric acid Sol: Hydrochloric acid is a conducting solution due to the presence of ions. When Neha passed the electric current through it, the bulb glowed indicating that the solution conducts electricity.
Q2: The solution labelled B would be (a) Sugar solution (b) Sulphuric acid solution (c) Sodium hydroxide (d) None of these Ans: (a) Sugar solution Sol: Sugar solution does not conduct electricity because it does not dissociate into ions in solution. Thus, the bulb did not glow when Neha replaced the solution with sugar solution.
Q3. Which of the following can be used to detect weak current flowing through the liquids in place of the torch bulb? (a) LED (Light emitting diode) (b) Compass (c) Both (a) and (b) (d) None of these Ans: (a) LED (Light emitting diode) Sol: An LED can be used to detect weak currents because it is sensitive to small amounts of current. A compass, on the other hand, is not used for detecting current directly.
Q4: When electric current is passed through a conducting solution, there is a change of colour of the solution. This indicates (a) Heating effect of electric current (b) Chemical effect of electric current (c) Magnetic effect of electric current (d) Lightning effect of electric current Ans: (b) Chemical effect of electric current Sol: The change in color of the solution indicates a chemical reaction occurring due to the electric current, which is the chemical effect of electric current.
Case Study 2
An electric current is passed through acidified water. Bubbles of oxygen gas and hydrogen gas are produced at the two electrodes immersed in it. Oxygen is formed at the positive electrode (anode) which is connected to the positive terminal of the battery and Hydrogen gas is formed at the negative electrode (cathode) which is connected to the negative terminal of the battery. The decomposition of acidified water into hydrogen and oxygen by passing an electric current is an example of electrolysis. Thus, electrolysis is the passage of electricity through a liquid or a solution accompanied by a chemical change.
Q1: Which of the following energy is used to decompose water into its elements? (a) Heat energy (b) Light energy (c) Chemical energy (d) Electrical energy Ans: (d) Electrical energy Sol: Electrolysis of water involves passing an electric current through the water to decompose it into hydrogen and oxygen gases. Thus, electrical energy is used in this process.
Q2: In the process of electrolysis, the current is carried out inside the electrolyte by (a) Electron (b) Atoms (c) Positive and negative ions (d) All of the above Ans: (c) Positive and negative ions Sol: During electrolysis, the electric current is carried through the electrolyte by the movement of positive and negative ions. Electrons move through the external circuit but do not carry the current inside the electrolyte.
Q3: Not pure water but acidic water is used for carrying out electrolysis because (a) Pure water is a good conductor of electricity (b) Pure water is a bad conductor of electricity (c) Pure water is hardly available (d) None of these Ans: (b) Pure water is a bad conductor of electricity Sol: Pure water does not conduct electricity well because it lacks sufficient ions. Acidic water, which contains dissolved acids, increases the conductivity of the water and allows the electric current to pass through more effectively.
Q4: Which of the following is the application of Electrolysis? (a) Extraction of metals (b) Purification of metals (c) Electroplating (d) All of these Ans: (d) All of these Sol: Electrolysis has several applications, including the extraction of metals from ores, purification of metals, and electroplating to coat surfaces with a layer of metal.
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Chapter: Chemical Effects of Current
Directions : Read the following and answer the questions given below:
Take out carbon rods carefully from two discarded cells. Clean their metal caps with sandpaper. Wrap copper wires around the metal caps of the carton rods to form electrodes and join them to a battery, Pour a cupful of water into a glass/plastic bowl. Add a teaspoonful of salt or a few drops of lemon juice to water to make it more conducting. Now immerse the electrodes in this solution. Make sure that the metal caps of the carbon rods are outside the water. Wait for 3-4 minutes, Observe the electrodes carefully.
Q(i) Which of the following is not a chemical effect of electric current?
(a) Bubbles of a gas may be formed on the electrodes.
(b) Deposits of metal may be seen on the electrodes.
(c) An electric bulb connected to the circuit may start glowing.
(d) Changes in the colour of solutions may occur.
Q(ii) Gas bubbles of ___________will be evolved near the electrodes in the given experiment.
(a) Oxygen and Hydrogen
(b) Carbon dioxide and Hydrogen
(c) Oxygen and Carbon dioxide
(d) None of these
Q(iii) A tester is used to check the conduction of electricity through two liquids, 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;
(a) liquid A is a better conductor than liquid B.
(b) liquid B is a better conductor than liquid A.
(c) both liquids are equally conducting.
(d) conducting properties of liquid cannot be compared in this manner.
Q(iv) Why do we clean the metal caps with sandpaper before using them?
(a) to make the metal caps smooth
(b) to make the metal cops appear bright and shiny
(c) to make the metal caps insulating
(d) to make the metal caps free of any rust
i. (a) gas bubbles may be formed on the electrodes.
ii. (a) Oxygen and Hydrogen
iii.(a) liquid A is a better conductor than liquid B.
iv. (d) to make the metal caps free of any rust
Ch 14 - Chemical Effects of Electric Current (NCERT Solutions) Ch 14 - Chemical Effects of Electric Current (MCQs) Ch 14 - Chemical Effects of Electric Current (Q & A) Ch 14 - Chemical Effects of Electric Current (Worksheet) Ch 14 - Chemical Effects of Electric Current (Worksheet-2)
CBSE All In One NCERT Based Science Class 8 2022-23 Edition
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