Calcium oxide reacts vigorously with water to produce slaked lime whose solution is used for white-washing walls. This slaked lime reacts with component (A) in air to form a thin layer of component (B) to give a shiny finish. What are the components (A) and (B)?
- A.A-O₂; B-CaCO₃
- B.A-CO₂; B-Ca(OH)₂
- C.A-O₂; B-Ca(OH)₂
- D.A-CO₂; B-CaCO₃
Correct answer
Explanation
The correct answer is D, A-CO₂; B-CaCO₃. Slaked lime on the wall slowly absorbs carbon dioxide from the air and turns into a thin, shiny layer of calcium carbonate.
Quicklime, CaO, reacts with water in a strongly exothermic reaction: CaO + H₂O → Ca(OH)₂. The product, calcium hydroxide or slaked lime, is applied to walls as a suspension. Over two or three days it reacts with atmospheric carbon dioxide: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is the same compound found in marble and limestone, which is why the whitewashed wall gets its hard, glossy finish. The same reaction turns clear lime water milky when carbon dioxide is bubbled through it.
Option A is wrong because oxygen does not react with slaked lime; carbon dioxide does. Option B is wrong because calcium hydroxide is what goes on the wall, not what forms there. Option C is wrong on both counts.
Exam tip: quicklime CaO, slaked lime Ca(OH)₂, limestone or marble CaCO₃; whitewash shines by taking CO₂ from the air.
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