Generated by claude-sonnet-4-6 · 2026-06-14 10:29 · grounding rag
Model Answer
(i) A few drops of acid (dilute H₂SO₄) are added to water to make it a good conductor of electricity (pure water is a poor conductor).
- Gas liberated at cathode: Hydrogen (H₂)
- Gas liberated at anode: Oxygen (O₂)
Relationship: Volume of H₂ at cathode : Volume of O₂ at anode = 2 : 1
(ii) When silver chloride (AgCl) is exposed to sunlight, it turns grey due to decomposition into silver (Ag) and chlorine (Cl₂).
$$2\text{AgCl} \xrightarrow{\text{sunlight}} 2\text{Ag} + \text{Cl}_2$$
Type of reaction: Photochemical decomposition reaction.
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Explanation
- In part (i), examiners expect three things: reason for adding acid, names of gases at each electrode, and the 2:1 volume ratio. Missing any one costs a mark.
- In part (ii), state the observable change (white → grey), write the balanced equation, and name the reaction type. "Photochemical decomposition" is the exact term expected — just "decomposition" may not fetch full credit.