Read the following and answer the questions that follow:
During whitewashing, calcium oxide (quick lime) is first mixed with water to produce calcium hydroxide (slaked lime). This reaction produces a large amount of heat. The slaked lime is then applied to walls. Over two to three days, calcium hydroxide slowly reacts with the carbon dioxide present in air to form a thin layer of calcium carbonate, which gives the walls a shiny finish. Interestingly, marble has the same chemical formula as this calcium carbonate layer.
(i) Write the balanced chemical equation for the reaction between calcium oxide and water. What type of reaction is it — exothermic or endothermic? (1 mark)
(ii) Write the balanced chemical equation for the reaction between calcium hydroxide and carbon dioxide. Name the type of reaction. (1 mark)
(iii) What is the chemical formula of marble, and how is it related to the product of whitewashing? (1 mark)
(iv) Name the broad category of chemical reaction (from types of reactions) that the first step — formation of slaked lime — belongs to. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 00:55 · grounding rag
Model Answer
(i)
$$\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq) + \text{Heat}$$
It is an exothermic reaction, as a large amount of heat is released.
(ii)
$$\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$$
It is a combination reaction, as two substances combine to form a single product.
(iii) The chemical formula of marble is CaCO₃. It is the same as the calcium carbonate layer formed on walls during whitewashing, showing they are chemically identical.
(iv) The formation of slaked lime belongs to the combination reaction category, where calcium oxide and water (two reactants) combine to form a single product, calcium hydroxide.
Source: Chapter 1, Section 1.2.1 – Combination Reaction
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Explanation
- (i) demands the balanced equation AND the type — don't forget "Heat" on the product side as shown in the textbook.
- (ii) The reaction is a combination reaction (two substances → one product); some students mistakenly call it a "neutralisation" — avoid that here.
- (iii) Simply state CaCO₃ and explain the link — the textbook explicitly says marble and the whitewash layer share the same formula.
- (iv) Even though it is also exothermic, the question asks for the "broad category of chemical reaction from types of reactions," which is combination reaction. Exothermic describes energy change, not the type of reaction.