Freshly whitewashed walls initially have a dull finish, but after two to three days they develop a shiny appe…

Question

Freshly whitewashed walls initially have a dull finish, but after two to three days they develop a shiny appearance. Explain the chemical reason for this change.

Accepted Answer

## Model Answer When walls are whitewashed, calcium hydroxide [Ca(OH)₂] is applied. Initially, it remains as a dull paste. Over two to three days, it reacts with carbon dioxide (CO₂) present in air to form calcium carbonate (CaCO₃): $$	ext{Ca(OH)}_2 + 	ext{CO}_2 ightarrow 	ext{CaCO}_3 + 	ext{H}_2	ext{O}$$ Calcium carbonate is a hard, shiny crystalline substance, which gives the wall its shiny appearance. This is an example of a combination reaction where CO₂ combines with Ca(OH)₂. *Source: Chapter 1, Chemical Reactions and Equations* --- ## Explanation - The key chemistry here is **Ca(OH)₂ + CO₂ → CaCO₃ + H₂O**; examiners expect this equation. - Mention that CaCO₃ is responsible for the shiny finish — that's the direct answer to "why shiny." - Identifying it as a **combination reaction** can fetch an extra mark, so include it. - Do not confuse with decomposition or displacement — this is strictly a combination/addition reaction involving atmospheric CO₂.

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