Q1. [1] § 1.2.3 Displacement Reaction
In the following experimental setup of electrolysis of water, if P and Q are the gases collected in the test tubes enclosing the electrodes R and S, then select the option/options in which the matching is correct :
(i) P – Oxygen gas , R – Anode
(ii) P – Hydrogen gas , R – Cathode
(iii) Q – Hydrogen gas , S – Cathode
(iv) Q – Oxygen gas , S – Anode
- (A) (i) and (ii)
- (B) (iii) and (iv)
- (C) (i) and (iii)
- (D) (ii) and (iv)
Previously asked in CBSE board exam
2025 31/6/1 Q1
Generated by claude-sonnet-4-6 · 2026-06-14 10:29 · grounding rag
Model Answer
(D) (ii) and (iv)
In electrolysis of water, hydrogen gas (double the volume) collects at the cathode and oxygen at the anode. From the diagram, P (left, more gas) is hydrogen → R is cathode; Q (right, less gas) is oxygen → S is anode. So (ii) and (iv) are correct.
Explanation
- At cathode: 2H⁺ + 2e⁻ → H₂; at anode: 2OH⁻ → H₂O + ½O₂. Hydrogen is produced in double the volume of oxygen.
- The test tube with more gas (P, left = R) collects H₂ → R is cathode. The test tube with less gas (Q, right = S) collects O₂ → S is anode.
- Statement (i) wrongly calls P oxygen; statement (iii) wrongly calls Q hydrogen. Only (ii) and (iv) are correct → Option (D).
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