Science (086) — AI-generated practice question

The reaction responsible for the hard, shiny finish is:

$$\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$$

Calcium hydroxide (slaked lime) reacts with carbon dioxide present in air to form calcium carbonate (CaCO₃), which gives the wall a hard, shiny finish. This is a combination reaction.

The finish takes two to three days to appear because calcium hydroxide reacts slowly with CO₂ in air. The calcium carbonate layer builds up gradually over this period before it becomes visible.

Source: Chapter 1, Section 1.2.1 – Combination Reaction

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• Examiners expect the balanced chemical equation — award it 1 mark.
• The reaction type (combination reaction) should be named — 1 mark.
• The reason for the delay ("reacts slowly with CO₂") must be stated explicitly — 1 mark.
• Do not confuse this with the preparation of slaked lime (CaO + H₂O); the whitewashing reaction is Ca(OH)₂ with CO₂.
• Note: CaCO₃ is also the chemical formula of marble — a nice fact but not required for marks here.