Chemical Effects of Electric Current — Class 8 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 8 Science chapter "Chemical Effects of Electric Current" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 8 Science chapter "Chemical Effects of Electric Current" — 8 important questions with detailed answers for…
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NCERT Solutions for Class 8 Science — Chemical Effects of Electric Current. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Chemical Effects of Electric Current — All 8 Questions Solved
Q1. What is electrolysis? Define it and give one example.
Electrolysis is the process of decomposing a chemical compound by passing electric current through it. The compound must be in liquid form (molten or solution). Example: Electrolysis of copper sulphate solution produces copper at the cathode and oxygen at the anode. Another example: Water electrolysis produces hydrogen and oxygen gases.
Q2. What happens at the anode and cathode during electrolysis of water?
Anode (positive electrode): Water is oxidized, producing oxygen gas and hydrogen ions. Cathode (negative electrode): Water is reduced, producing hydrogen gas and hydroxide ions. Overall: 2H₂O → 2H₂ (at cathode) + O₂ (at anode). Hydrogen gas is twice the volume of oxygen.
Q3. Define electroplating. Why is it used?
Electroplating is a process of coating a thin layer of one metal onto another metal using electrolysis. Uses: (1) Protect metals from corrosion, (2) Enhance appearance with shiny coating, (3) Prevent rust on iron objects, (4) Improve durability. Objects are dipped in electrolyte solution with the coating metal as anode.
Q4. What are electrolytes? Give two examples.
Electrolytes are substances that conduct electric current in their molten or aqueous solution form. They contain ions that allow current flow. Examples: (1) Copper sulphate solution, (2) Dilute hydrochloric acid, (3) Sodium chloride solution, (4) Battery acid (sulfuric acid). Non-electrolytes like sugar solution do not conduct electricity.
Q5. Why is salt solution a good electrolyte but pure water is a poor electrolyte?
Salt solution contains ions (Na⁺ and Cl⁻) that conduct electric current. Pure water has very few ions and is a poor conductor. When salt dissolves in water, it dissociates into ions, increasing conductivity. Pure water has only H⁺ and OH⁻ ions in trace amounts, making it non-conducting.
Q6. What is the role of an electrolyte in a battery?
The electrolyte in a battery allows ions to flow between the anode and cathode inside the battery. This ion flow completes the internal circuit, enabling electrons to flow through the external circuit. Without an electrolyte, no current can be generated. The chemical reaction in the electrolyte produces the electric energy.
Q7. Explain the process of electroplating an object with silver.
Setup: Object to be plated (cathode) and pure silver (anode) are immersed in silver salt solution (electrolyte). Electric current is passed: Silver ions move to the cathode and are reduced to silver metal, coating the object. Anode dissolves, replenishing silver ions in solution. Result: Thin layer of silver deposits on the object.
Q8. What are the products of electrolysis of copper sulphate solution using copper electrodes?
Cathode: Copper is deposited (Cu²⁺ + 2e⁻ → Cu). Anode: Copper dissolves (Cu → Cu²⁺ + 2e⁻). Result: Copper transfers from anode to cathode. No oxygen or hydrogen is produced. The anode decreases in mass while the cathode gains mass. This is used to purify copper metal.
Chemical Effects of Electric Current — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following liquids is a good conductor of electricity?
- A. Distilled water
- B. Sugar solution
- C. Lemon juice
- D. Kerosene oil
Answer: C. Lemon juice — Lemon juice contains citric acid, which dissociates into ions in water, making it a good conductor of electricity. Distilled water, sugar solution, and kerosene oil do not have sufficient ions to conduct electricity.
MCQ 2. Why is an LED preferred over a normal bulb to test the conductivity of some liquids?
- A. LEDs are brighter
- B. LEDs consume less current
- C. LEDs are cheaper
- D. LEDs can glow even with a very weak current
Answer: D. LEDs can glow even with a very weak current — An LED requires a very small current to glow, making it suitable for detecting weak currents in liquids that are poor conductors. A normal bulb might not glow under such conditions.
MCQ 3. When electric current is passed through an aqueous solution of sodium chloride, what is observed at the electrodes?
- A. Bubbles of gas
- B. Metal deposition
- C. Solution color change
- D. No observable change
Answer: A. Bubbles of gas — Passing electric current through an aqueous solution of sodium chloride causes the decomposition of water and chloride ions. Hydrogen gas is produced at the cathode and chlorine gas at the anode, appearing as bubbles.
MCQ 4. The process of depositing a layer of any desired metal on another material by means of electricity is called:
- A. Magnetic effect
- B. Heating effect
- C. Chemical effect
- D. Electroplating
Answer: D. Electroplating — Electroplating is a specific application of the chemical effects of electric current where a thin layer of one metal is deposited onto another object using an electric current.
MCQ 5. Distilled water does not conduct electricity, but tap water does. What is the reason for this difference?
- A. Tap water has a higher temperature
- B. Distilled water is purer and lacks dissolved salts
- C. Tap water contains more oxygen
- D. Distilled water is denser
Answer: B. Distilled water is purer and lacks dissolved salts — Distilled water is pure and contains no dissolved salts or minerals, which provide the ions necessary for conducting electricity. Tap water contains dissolved salts and impurities that dissociate into ions, enabling it to conduct current.
MCQ 6. To electroplate an iron spoon with silver, which of the following set-up is correct?
- A. Iron spoon at anode, silver at cathode, silver nitrate solution as electrolyte.
- B. Iron spoon at cathode, silver at anode, silver nitrate solution as electrolyte.
- C. Iron spoon at anode, silver at cathode, iron sulphate solution as electrolyte.
- D. Iron spoon at cathode, silver at anode, iron nitrate solution as electrolyte.
Answer: B. Iron spoon at cathode, silver at anode, silver nitrate solution as electrolyte. — In electroplating, the object to be plated (iron spoon) is made the cathode, the plating metal (silver) is made the anode, and the electrolyte is a salt solution of the plating metal (silver nitrate).
MCQ 7. Which of the following is an example of an electrolyte?
- A. Alcohol
- B. Pure water
- C. Salt solution
- D. Vegetable oil
Answer: C. Salt solution — A salt solution contains dissolved ionic compounds that dissociate into free ions, allowing it to conduct electricity and act as an electrolyte. Alcohol, pure water, and vegetable oil are generally non-electrolytes.
MCQ 8. When electric current is passed through water, which gas is usually collected at the cathode?
- A. Oxygen
- B. Hydrogen
- C. Nitrogen
- D. Carbon dioxide
Answer: B. Hydrogen — During the electrolysis of water, hydrogen gas is produced at the cathode (negative electrode) and oxygen gas is produced at the anode (positive electrode).
MCQ 9. Why are car bumpers and bicycle handlebars often electroplated with chromium?
- A. To make them lighter
- B. To make them stronger
- C. To prevent corrosion and give a shiny appearance
- D. To make them cheaper
Answer: C. To prevent corrosion and give a shiny appearance — Chromium plating is done because chromium is resistant to scratches and corrosion, and it provides a bright, shiny appearance to the object, enhancing its aesthetic appeal and durability.
MCQ 10. The effect of electric current that causes chemical changes in a conducting liquid is known as the:
- A. Heating effect
- B. Magnetic effect
- C. Chemical effect
- D. Mechanical effect
Answer: C. Chemical effect — When electric current passes through certain liquids, it can cause chemical reactions, leading to the formation of bubbles, metal deposits, or changes in solution color. This is known as the chemical effect of electric current.
MCQ 11. To test if a liquid is a very poor conductor of electricity, what modification can be made to a simple circuit with a bulb?
- A. Use a thicker wire
- B. Increase the voltage
- C. Replace the bulb with an LED
- D. Use a smaller battery
Answer: C. Replace the bulb with an LED — An LED requires a much smaller current to glow compared to a conventional bulb. Therefore, replacing the bulb with an LED allows the detection of even very weak currents passing through poor conductors.
MCQ 12. Which scientist is credited with establishing the laws of electrolysis?
- A. Alessandro Volta
- B. Michael Faraday
- C. Thomas Edison
- D. James Watt
Answer: B. Michael Faraday — Michael Faraday performed extensive experiments on the chemical effects of electric current and formulated the fundamental laws of electrolysis, which describe the quantitative relationships between electricity and chemical change.
MCQ 13. Which of these substances is an insulator?
- A. Copper
- B. Saltwater
- C. Rubber
- D. Graphite
Answer: C. Rubber — Rubber does not allow electric current to pass through it easily, making it an excellent electrical insulator. Copper and graphite are conductors, and saltwater is an electrolyte.
MCQ 14. How do most liquids conduct electricity?
- A. Through free electrons
- B. Through the movement of atoms
- C. Through the movement of ions
- D. Through vibrations of molecules
Answer: C. Through the movement of ions — In most conducting liquids (electrolytes), electricity is conducted by the movement of charged particles called ions towards the oppositely charged electrodes.
MCQ 15. What is a major environmental concern associated with electroplating factories?
- A. High energy consumption
- B. Noise pollution
- C. Disposal of used conducting solutions
- D. Air pollution from fumes
Answer: C. Disposal of used conducting solutions — Electroplating processes use highly toxic chemicals in their solutions, such as cyanides and heavy metal salts. Improper disposal of these used solutions can severely pollute water bodies and soil.
MCQ 16. What is the primary purpose of electroplating zinc on iron objects?
- A. To make them shine
- B. To make them heavier
- C. To prevent rusting
- D. To improve grip
Answer: C. To prevent rusting — Electroplating iron with zinc (galvanization) protects the iron from rusting. Zinc is more reactive than iron and acts as a sacrificial layer, corroding instead of the iron.
MCQ 17. Which of the following pairs correctly identifies an electrolyte and a non-electrolyte?
- A. Lemon juice (electrolyte), Kerosene (non-electrolyte)
- B. Vinegar (non-electrolyte), Tap water (electrolyte)
- C. Salt solution (non-electrolyte), Sugar solution (electrolyte)
- D. Copper (electrolyte), Plastic (non-electrolyte)
Answer: A. Lemon juice (electrolyte), Kerosene (non-electrolyte) — Lemon juice contains acids that form ions, making it an electrolyte. Kerosene is an organic compound that does not dissociate into ions and is a non-electrolyte.
MCQ 18. If two electrodes are dipped in copper sulphate solution and an electric current is passed, what will be deposited on the cathode?
- A. Copper
- B. Sulphur
- C. Oxygen
- D. Hydrogen
Answer: A. Copper — In the electrolysis of copper sulphate solution, copper ions (Cu²⁺) are attracted to the negatively charged cathode and gain electrons to form solid copper metal, which gets deposited.
MCQ 19. In electroplating, the object to be electroplated is always connected to the:
- A. Positive terminal of the battery
- B. Negative terminal of the battery
- C. Either terminal
- D. Ground
Answer: B. Negative terminal of the battery — The object to be plated acts as the cathode, which is connected to the negative terminal of the battery. Positive metal ions from the electrolyte are attracted to the cathode and get deposited.
MCQ 20. Adding a pinch of common salt to distilled water makes it a conductor of electricity. This is because:
- A. Salt increases the density of water
- B. Salt reacts with water to form a new compound
- C. Salt dissociates into ions in water
- D. Salt reduces the boiling point of water
Answer: C. Salt dissociates into ions in water — Common salt (sodium chloride) is an ionic compound. When dissolved in water, it dissociates into sodium ions (Na⁺) and chloride ions (Cl⁻), which are free to move and conduct electricity.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.