Chapter Overview & Weightage
Chemical Effects of Electric Current is a Class 8 Science chapter that connects electricity to chemistry — specifically electrolysis. It explains how electric current can pass through liquids and cause chemical changes, which is the basis for electroplating, purification of metals, and electrolytic refining.
In CBSE Class 8 annual exams, this chapter typically carries 6–8 marks. Questions include: defining electrolytes and non-electrolytes, explaining electroplating with diagram, giving examples of uses of electroplating, and the LED/bulb experiment to test conductivity. Diagrams are frequently tested.
Key Concepts You Must Know
1. Conductors and Insulators in Liquids:
- Most liquids are poor conductors — pure water, oil, sugar solution don’t conduct electricity
- Electrolytes: Liquids (or their solutions) that conduct electricity and undergo chemical change. Examples: salt water (NaCl solution), lemon juice, vinegar, copper sulphate solution, HCl solution
- Non-electrolytes: Don’t conduct electricity. Examples: pure water, sugar solution, cooking oil, alcohol
2. Testing Conductivity: A simple circuit with a bulb (or LED) and two electrodes can test if a liquid conducts. If the bulb glows, the liquid conducts. LED is more sensitive — it glows even with weak currents.
3. Electrolysis: When electric current passes through an electrolyte:
- The liquid conducts electricity
- Chemical changes occur at the electrodes
- Decomposition of the electrolyte occurs
Example: Electrolysis of water ()
4. Electrodes:
- Anode (+): electrode connected to positive terminal of battery. Oxidation occurs here.
- Cathode (−): electrode connected to negative terminal. Reduction occurs here.
5. Electroplating: Depositing a thin layer of a metal on an object using electrolysis.
Applications of electroplating:
- Chromium plating on car parts (prevents rust, shiny look)
- Gold/silver plating on jewellery and decorative items
- Zinc plating (galvanisation) on iron/steel to prevent rusting
- Tin plating on food cans (prevents rusting, non-toxic)
- Nickel plating for hardness
Important Formulas
- Electrolyte: Copper sulphate (CuSO₄) solution
- Anode: Copper plate (dissolves to replenish Cu²⁺)
- Cathode: Object to be plated
- Result: Copper deposits on cathode
Cathode reaction: Cu²⁺ + 2e⁻ → Cu (copper deposited) Anode reaction: Cu → Cu²⁺ + 2e⁻ (copper dissolved)
Solved Previous Year Questions
PYQ 1 — Explain Electroplating with Diagram (5 marks)
Answer:
Electroplating is the process of depositing a layer of a desired metal on an object using electrolysis.
How it works for chromium plating on iron:
- The object to be plated (iron) is made the cathode (connected to negative terminal)
- A chromium plate is made the anode (connected to positive terminal)
- A solution containing chromium ions (chromic acid) is used as the electrolyte
- When current flows, chromium ions (Cr³⁺) in the electrolyte move to the cathode and deposit as metallic chromium on the iron object
- The anode chromium slowly dissolves to replenish the electrolyte
Diagram elements to include:
- Battery with + and - terminals
- Anode (chromium) connected to +
- Cathode (iron object) connected to -
- Electrolyte (chromic acid/chrome sulphate solution)
- Arrow showing current direction
- Label showing chromium deposition at cathode
PYQ 2 — Give three uses of electroplating (3 marks)
Answer:
- Preventing rusting: Zinc plating (galvanisation) on iron roofing sheets and dustbins — zinc forms a protective layer that doesn’t rust.
- Decoration and value: Gold and silver plating on jewellery, trophies, and decorative items gives them an attractive appearance at lower cost than solid gold/silver.
- Corrosion resistance in food cans: Tin plating on steel food cans — tin doesn’t rust and is non-toxic, making it safe for storing food.
PYQ 3 — Why does pure water not conduct electricity? (2 marks)
Answer: Pure water is a covalent compound with very few free ions. For a liquid to conduct electricity, it must have freely moving charged particles (ions). Pure water has negligible ionic concentration (only mol/L of H⁺ and OH⁻ ions), so it cannot conduct electricity in any practical sense.
In contrast, salt water (NaCl dissolved in water) provides Na⁺ and Cl⁻ ions that move freely and carry electric current.
Difficulty Distribution
| Difficulty | Topics | Approximate % |
|---|---|---|
| Easy | Conductors/non-conductors of electricity, examples | 40% |
| Medium | Electrolysis definition, electrodes, simple questions on electroplating | 40% |
| Hard | Detailed mechanism of electroplating, process and diagram | 20% |
Expert Strategy
Draw neat diagrams. Electroplating diagrams are worth 2 marks in most papers. Label: battery, switch, anode (+), cathode (−), electrolyte, and direction of ion movement.
Examples of good and bad conductors: Remember at least 3 of each. Good conductors: NaCl solution, copper sulphate, lemon juice, vinegar, tap water. Poor conductors: sugar solution, pure water, cooking oil, distilled water.
Electroplating uses: Know at least 5 practical applications with the metal used and the reason for plating.
The key phrase to use in answers: “Electroplating is used to prevent corrosion/add beauty/improve wear resistance.” Always give both the metal plated AND the reason in your answer for full marks.
Common Traps
Trap 1: Saying pure water conducts electricity. It does not in practical terms. Tap water (which contains dissolved salts) conducts weakly. The distinction between pure water, tap water, and salt water is specifically tested.
Trap 2: Mixing up anode and cathode. Cathode is always connected to the negative terminal. The object to be plated is always the cathode (where deposition occurs). Use the mnemonic: “Cathode = Connected to negative; deposition (comes) to cathode.”
Trap 3: Saying electrolysis always decomposes water into hydrogen and oxygen. That’s only for water electrolysis. For copper sulphate solution, copper deposits at the cathode and oxygen is released at the anode (not hydrogen). The products depend on the electrolyte used.