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Class 8 Science Chapter 11 Chemical Effects Of Electric Current

Chapter 11 of Class 8 Science, titled "Chemical Effects of Electric Current," explores the relationship between electricity and chemical reactions. In this chapter, students learn how electric current can cause chemical changes, a phenomenon known as electrolysis. The chapter introduces the concept of conducting solutions and how they allow electric current to flow, leading to chemical reactions at the electrodes. Key concepts such as electroplating, the process of coating one metal with another using electric current, are discussed, along with real-life applications like the plating of jewelry and utensils. The chapter also highlights the dangers of electric current in conducting liquids and the importance of safety measures when dealing with electricity. Students will also understand the role of electrolytes, the substances that facilitate the conduction of electricity in solutions. The quiz on this chapter will assess students' comprehension of how electric current can bring about chemical changes, the applications of electrolysis, and the importance of using electricity safely in various situations. By the end of this chapter, students will gain a deeper insight into the fascinating chemical effects of electric current.

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Category: Introduction

1. A car accelerates uniformly from rest to a speed of 72 km/h in 10 seconds. What is the acceleration of the car in m/s$^2$?

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Category: Introduction

2. (A) The gravitational force between two objects is always attractive.
(R) Gravitational force is a result of the mass of objects and the distance between them.

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Category: Introduction

3. A train travels 300 km in 5 hours. If the speed of the train is increased by 20 km/h, how much time will it take to travel the same distance?

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Category: Importance of electrical safety (e.g., wet hands and electrical appliances)

4. Why is it dangerous to touch electrical appliances with wet hands?

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Category: Importance of electrical safety (e.g., wet hands and electrical appliances)

5. Which of the following actions should be avoided to ensure electrical safety?

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Category: Importance of electrical safety (e.g., wet hands and electrical appliances)

6. Which of the following materials does NOT conduct electricity?

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Category: Conductors and insulators:

7. (A) Metals like copper and aluminium are good conductors of electricity.
(R) Metals have free electrons that allow the flow of electric current.

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Category: Conductors and insulators:

8. A student tests the conductivity of distilled water and finds that it does not conduct electricity. However, when a pinch of common salt is added to the same distilled water, the solution conducts electricity. What is the most likely reason for this change in conductivity?

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Category: Conductors and insulators:

9. (A) Lemon juice conducts electricity because it contains ions that facilitate the flow of electric current.
(R) The presence of free-moving ions in a liquid allows it to conduct electricity.

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Category: Good Conductors: Allow electricity to pass (e.g., metals like copper, aluminum).

10. Which type of water is a good conductor of electricity?

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Category: Good Conductors: Allow electricity to pass (e.g., metals like copper, aluminum).

11. What happens to the conductivity of distilled water when a pinch of common salt is added to it?

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Category: Good Conductors: Allow electricity to pass (e.g., metals like copper, aluminum).

12. (A) Tap water conducts electricity because it contains dissolved salts.
(R) The presence of dissolved salts in tap water makes it a good conductor of electricity.

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Category: Poor Conductors: Do not allow electricity to pass (e.g., plastic, wood, rubber).

13. Which of the following materials is a poor conductor of electricity?

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Category: Poor Conductors: Do not allow electricity to pass (e.g., plastic, wood, rubber).

14. A student tests the conductivity of three solutions: lemon juice dissolved in distilled water, sugar dissolved in distilled water, and salt dissolved in distilled water. Which of the following correctly ranks these solutions based on their conductivity?

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Category: Poor Conductors: Do not allow electricity to pass (e.g., plastic, wood, rubber).

15. What happens when common salt is dissolved in distilled water?

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Category: Testing solid materials for conductivity.

16. (A) The deflection of a compass needle in a magnetic tester setup indicates the presence of an electric current in the liquid being tested.
(R) The magnetic effect of electric current causes the compass needle to deflect when current flows through a conductor.

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Category: Testing solid materials for conductivity.

17. (A) Conductivity of a material can be tested using a simple circuit with a bulb.
(R) A bulb in the circuit will glow if the material conducts electricity.

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Category: Testing solid materials for conductivity.

18. A student tests the conductivity of distilled water and finds that the bulb does not glow. What could be the reason for this observation?

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Category: Do Liquids Conduct Electricity?

19. A student tests the conductivity of distilled water using a tester and finds that it does not conduct electricity. When a pinch of common salt is dissolved in the same distilled water, the tester shows that the liquid now conducts electricity. What is the most likely explanation for this observation?

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Category: Do Liquids Conduct Electricity?

20. What happens when an electric current passes through a conducting solution?

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Category: Do Liquids Conduct Electricity?

21. Why does tap water conduct electricity while distilled water does not?

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Category: Testing electrical conductivity of liquids using a tester

22. Which of the following is classified as a poor conductor of electricity?

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Category: Testing electrical conductivity of liquids using a tester

23. Which of the following liquids would most likely cause a deflection in the compass needle of a magnetic needle tester when tested for electrical conductivity?

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Category: Testing electrical conductivity of liquids using a tester

24. What happens to the conductivity of distilled water when a pinch of common salt is dissolved in it?

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Category: Bulb-based tester setup

25. If a bulb-based tester is used to test the conductivity of lemon juice, and the bulb does not glow, what could be the possible reason?

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Category: Bulb-based tester setup

26. (A) If the bulb in the tester glows, it indicates that the liquid between the ends of the tester conducts electricity.
(R) The glowing of the bulb is due to the heating effect of the electric current passing through the filament.

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Category: Bulb-based tester setup

27. Why is an LED preferred over a bulb in a tester setup for testing weak electric currents?

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Category: Compass needle tester

28. A student performs an experiment using a compass needle tester to determine the conductivity of tap water and vegetable oil. The student notices that the compass needle deflects slightly for tap water but not for vegetable oil. What conclusion can be drawn from this observation?

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Category: Compass needle tester

29. When using a compass needle tester to check the conductivity of lemon juice, what observation would you expect if lemon juice is a good conductor of electricity?

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Category: Compass needle tester

30. Which of the following liquids is classified as a poor conductor based on the compass needle tester experiment?

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Category: LED-based tester for weak currents

31. A student tests the conductivity of distilled water using an LED-based tester. Initially, the LED does not glow. The student then adds a pinch of common salt to the distilled water and tests it again. What is the most likely outcome?

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Category: LED-based tester for weak currents

32. What happens if you connect the free ends of a tester together for more than a few seconds?

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Category: LED-based tester for weak currents

33. (A) An LED glows when a weak electric current flows through it.
(R) LEDs are designed to operate with very low currents and emit light even when the current is minimal.

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Category: Factors affecting conductivity of liquids

34. (A) Distilled water does not conduct electricity because it lacks free ions.
(R) Most liquids that conduct electricity are solutions of acids, bases, and salts, which provide free ions for conduction.

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Category: Factors affecting conductivity of liquids

35. (A) Distilled water is a poor conductor of electricity.
(R) Distilled water does not contain any dissolved salts or ions.

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Category: Factors affecting conductivity of liquids

36. A student prepares three solutions: Solution A by adding a few drops of lemon juice to distilled water, Solution B by adding a few drops of caustic soda to distilled water, and Solution C by adding sugar to distilled water. Which of the following statements is correct regarding the conductivity of these solutions?

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Category: Strong conductors: Lemon juice, vinegar, saltwater

37. (A) Lemon juice is a good conductor of electricity.
(R) Lemon juice contains acids that dissociate into ions, allowing the flow of electric current.

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Category: Strong conductors: Lemon juice, vinegar, saltwater

38. Which of the following liquids is a good conductor of electricity?

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Category: Strong conductors: Lemon juice, vinegar, saltwater

39. (A) Lemon juice is a good conductor of electricity because it contains citric acid.
(R) Acids, when dissolved in water, dissociate into ions which facilitate the flow of electric current.

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Category: Poor conductors: Distilled water, sugar solution, oil

40. Why does tap water conduct electricity while distilled water does not?

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Category: Poor conductors: Distilled water, sugar solution, oil

41. What happens when an electric current passes through a conducting solution?

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Category: Poor conductors: Distilled water, sugar solution, oil

42. (A) Distilled water is a poor conductor of electricity because it lacks free ions.
(R) The presence of free ions in a solution is essential for the conduction of electricity.

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Category: Table of Good/Poor Conducting Liquids

43. (A) Lemon juice is a good conductor of electricity.
(R) Lemon juice contains acids which facilitate the conduction of electricity.

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Category: Table of Good/Poor Conducting Liquids

44. When the free ends of a tester are dipped into a solution and the magnetic needle shows deflection, what does it indicate?

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Category: Table of Good/Poor Conducting Liquids

45. Which of the following liquids is a good conductor of electricity?

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Category: Conductors: Lemon juice, vinegar, salt solution

46. Which of the following liquids is a good conductor of electricity?

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Category: Conductors: Lemon juice, vinegar, salt solution

47. Why does distilled water not conduct electricity?

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Category: Conductors: Lemon juice, vinegar, salt solution

48. What happens to the conductivity of distilled water when a pinch of common salt is added?

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Category: Poor Conductors: Honey, vegetable oil, distilled water

49. What happens to the electrical conductivity of distilled water when common salt is added to it?

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Category: Poor Conductors: Honey, vegetable oil, distilled water

50. (A) Distilled water is a poor conductor of electricity.
(R) Distilled water does not contain any dissolved salts or impurities.

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Category: Poor Conductors: Honey, vegetable oil, distilled water

51. (A) Distilled water is a poor conductor of electricity.
(R) Distilled water lacks dissolved salts or ions that are necessary for electrical conductivity.

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Category: Chemical Effects of Electric Current

52. What is a major concern regarding the disposal of used conducting solutions in electroplating factories?

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Category: Chemical Effects of Electric Current

53. In electroplating factories, why is the disposal of used conducting solutions a major concern?

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Category: Chemical Effects of Electric Current

54. What gas is produced at the electrode connected to the positive terminal of the battery when electric current is passed through water?

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Category: Electrolysis: Decomposition of a substance due to passing electric current

55. A student performs an experiment where they pass an electric current through a copper sulphate solution using two electrodes. After some time, one electrode shows a significant increase in mass while the other does not. What could be the reason for this observation?

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Category: Electrolysis: Decomposition of a substance due to passing electric current

56. What happens when electric current is passed through copper sulphate solution using copper electrodes?

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Category: Electrodes & Electrolyte

57. What is the purpose of adding dilute sulphuric acid to the copper sulphate solution in electroplating?

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Category: Electrodes & Electrolyte

58. Boojho inserted copper wires of a tester into a potato and left them for half an hour. He observed a greenish-blue spot around one wire but not the other. Which terminal of the battery was the wire with the greenish-blue spot connected to?

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Category: Electrodes: Conductors in an electrolyte (e.g., carbon rods, metal plates)

59. During the electroplating of a metal spoon with silver, which electrode should the spoon be connected to in order to get a silver coating?

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Category: Electrodes: Conductors in an electrolyte (e.g., carbon rods, metal plates)

60. (A) Carbon rods can be used as electrodes in conducting solutions.
(R) Carbon is a good conductor of electricity.

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Category: Electrolyte: Conducting liquid (e.g., saltwater, acid solutions)

61. A conducting solution of hydrochloric acid is subjected to electrolysis using platinum electrodes. What products are formed at the electrodes?

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Category: Electrolyte: Conducting liquid (e.g., saltwater, acid solutions)

62. What type of gas bubbles are formed on the electrode connected to the positive terminal of the battery when an electric current is passed through water?

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Category: Observations in Conducting Solutions

63. During an electroplating experiment, a student uses copper sulphate solution with a copper anode and an iron spoon as the cathode. What will be observed at the cathode after passing an electric current through the solution?

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Category: Observations in Conducting Solutions

64. When an electric current is passed through a copper sulphate solution, copper gets deposited on the electrode connected to which terminal of the battery?

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Category: Formation of gas bubbles near electrodes (e.g., electrolysis of water)

65. Which electrode produces hydrogen gas during the electrolysis of water?

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Category: Formation of gas bubbles near electrodes (e.g., electrolysis of water)

66. If during the electrolysis of water, the volume of oxygen gas collected at the anode is 56 mL at STP, what volume of hydrogen gas would be collected at the cathode under the same conditions?

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Category: Deposition of metals on electrodes

67. A copper electrode is connected to the negative terminal of a battery and another copper electrode is connected to the positive terminal in a copper sulphate solution. If 0.5 grams of copper is deposited on the cathode, how much copper will dissolve from the anode?

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Category: Deposition of metals on electrodes

68. A food storage container is made by electroplating tin onto iron. Why is tin preferred over other metals for this application?

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Category: Change in color of the solution

69. Which metal is commonly used for electroplating iron to prevent rusting?

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Category: Change in color of the solution

70. During the electrolysis of potassium iodide solution using platinum electrodes, iodine is formed at one electrode. What change in the solution’s color would you observe?

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Category: William Nicholson’s Discovery (1800)

71. (A) In William Nicholson’s experiment, oxygen gas is produced at the electrode connected to the positive terminal of the battery.
(R) The passage of electric current through water causes electrolysis, resulting in the decomposition of water into hydrogen and oxygen gases.

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Category: William Nicholson’s Discovery (1800)

72. What observation would indicate a chemical effect of electric current in a conducting solution?

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Category: Electrolysis of Water: Formation of Oxygen and Hydrogen

73. During the electrolysis of water, if a current of 5 amperes is passed through the solution for 1 hour, how many grams of hydrogen gas will be produced at the cathode? (Given: Faraday's constant = 96500 C/mol)

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Category: Electrolysis of Water: Formation of Oxygen and Hydrogen

74. (A) During the electrolysis of water, oxygen gas is formed at the anode.
(R) The anode is connected to the positive terminal of the battery, and oxygen is produced due to the oxidation of water molecules.

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Category: Electroplating

75. (A) Chromium plating is used to protect iron from rusting.
(R) Chromium is resistant to corrosion and scratches.

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Category: Electroplating

76. (A) Electroplating is primarily used to enhance the aesthetic appearance of objects.
(R) Chromium plating is done on car parts and bicycle handlebars because chromium has a shiny appearance and resists scratches.

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Category: Definition: Coating an object with a thin layer of metal using electricity

77. (A) Electroplating is used to deposit a layer of chromium on car parts to enhance their appearance and prevent corrosion.
(R) Chromium is a metal that does not corrode and has a shiny appearance, making it suitable for electroplating.

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Category: Definition: Coating an object with a thin layer of metal using electricity

78. (A) Chromium plating is used on car parts because chromium is cheaper than other metals.
(R) Chromium is chosen for electroplating due to its shiny appearance, resistance to corrosion, and scratch resistance.

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Category: Electroplating Experiment (Activity 11.7)

79. What would happen if the electrodes are interchanged in the electroplating setup after the initial process?

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Category: Electroplating Experiment (Activity 11.7)

80. Why is the disposal of used conducting solutions a major concern in electroplating factories?

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Category: Copper plates, copper sulfate solution, battery

81. In electroplating, which terminal of the battery does the copper get deposited on when copper sulphate solution is used?

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Category: Copper plates, copper sulfate solution, battery

82. In an electroplating setup using copper sulphate solution and two copper plates of size 10 cm × 4 cm, if the current is passed for 30 minutes and 0.5 grams of copper is deposited on the cathode, how much copper would have been dissolved from the anode?

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Category: Copper deposition on the negative electrode

83. (A) During electroplating, copper is deposited on the negative electrode because it is connected to the negative terminal of the battery.
(R) The free copper ions in the solution are attracted to the negative electrode due to their positive charge.

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Category: Copper deposition on the negative electrode

84. If a carbon rod is used instead of a copper plate as the negative electrode in a copper electroplating setup, what will be observed regarding the copper coating?

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Category: Mechanism of Electroplating

85. (A) In the process of electroplating, copper is transferred from one electrode to another through the solution.
(R) Copper ions from the solution are deposited on the negative electrode, while an equal amount of copper is dissolved from the positive electrode.

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Category: Mechanism of Electroplating

86. Why is chromium plating commonly used on car parts and bicycle handlebars?

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Category: Metal ions in solution move towards the negative electrode

87. In an electroplating process using copper sulphate solution, why is a few drops of dilute sulphuric acid added to the solution?

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Category: Metal ions in solution move towards the negative electrode

88. In an electroplating setup using copper sulphate solution, if the negative electrode is replaced with a carbon rod, what will happen to the carbon rod after passing electric current?

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Category: The positive electrode dissolves metal into the solution

89. (A) During electroplating, the positive electrode (anode) dissolves into the solution to replenish the metal ions lost during the deposition process.
(R) The dissolution of the anode ensures that the concentration of metal ions in the solution remains constant, which is essential for continuous electroplating.

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Category: The positive electrode dissolves metal into the solution

90. In an electroplating setup using copper sulphate solution, if the positive electrode is made of iron instead of copper, what will happen to the iron electrode over time?

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Category: Practical Applications of Electroplating

91. What is a major health concern associated with chromium electroplating, and how can it be mitigated?

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Category: Practical Applications of Electroplating

92. What is the primary reason for using chromium plating on car parts?

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Category: Chromium plating (car parts, taps, cycle handles) – prevents rusting

93. Which of the following is NOT a benefit of chromium plating?

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Category: Chromium plating (car parts, taps, cycle handles) – prevents rusting

94. Why is chromium plating commonly used on car parts and cycle handles?

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Category: Gold and Silver plating (ornaments) – makes jewelry affordable

95. In an electroplating setup, if 2.5 g of silver is deposited on an ornament in 1 hour, what current was used? (Atomic mass of silver = 108 g/mol, valency = 1, Faraday's constant = 96500 C/mol)

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Category: Gold and Silver plating (ornaments) – makes jewelry affordable

96. Which of the following metals is commonly used for electroplating in jewelry to give it a golden appearance?

97 / 100

Category: Tin plating on iron cans – prevents food contamination

97. In the process of tin plating on iron cans, what is the primary reason for using tin instead of a more reactive metal?

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Category: Tin plating on iron cans – prevents food contamination

98. Why is tin used in the electroplating of iron cans for food storage?

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Category: Zinc plating on iron (Galvanization) – protects from rusting

99. Which of the following metals is commonly used in electroplating for its shiny appearance and resistance to scratches?

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Category: Zinc plating on iron (Galvanization) – protects from rusting

100. (A) Zinc plating on iron is done to protect it from corrosion.
(R) Zinc acts as a sacrificial anode, preventing the iron from rusting.

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