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2017 O Level Chem P1 Q34 — Acids and Bases / Neutralisation Solution

Acids and Bases / Neutralisation 2017 O Level Pure Chemistry · Paper 1 · Q34

Question

Acid rain contains sulfuric acid and can cause lakes to become acidic. Acidic lakes may be treated with powdered limestone (impure calcium carbonate) to neutralise the acidity. Calcium sulfate is formed. If large lumps of limestone are used, instead of powdered limestone, the reaction starts but soon stops, leaving most of the limestone unreacted. Which statement explains why the reaction starts but soon stops?

AA layer of insoluble calcium sulfate forms on the surface of the lumps.
BLimestone only contains small amounts of calcium carbonate.
CPowdered limestone is more reactive than lumps of limestone.
DThe acid reacts with the calcium sulfate instead of the calcium carbonate.

Correct answer: A

Video Solution

Explanation

Answer: A. The calcium sulfate formed is insoluble and coats the surface of the limestone lumps, sealing them from the acid so the reaction soon stops.

Powdered limestone works because it exposes far more surface area. B and C do not explain why the reaction stops, and D is not what happens.

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