Consider the following sequence: calcium carbonate is heated strongly → the product (CaO) is mixed with water → the resulting solution (lime water) is exposed to air for several days.
(a) Write balanced chemical equations for each of the three steps.
(b) Name the type of reaction occurring in the first step and in the third step.
(c) The second step causes the container to become very hot. Identify whether this step is exothermic or endothermic, and explain why this energy behaviour is consistent with the type of reaction it represents.
Generated by claude-sonnet-4-6 · 2026-06-26 00:56 · grounding rag
Model Answer
(a) Balanced chemical equations:
Step 1 (Heating calcium carbonate):
$$\text{CaCO}_3(s) \xrightarrow{\text{Heat}} \text{CaO}(s) + \text{CO}_2(g)$$
Step 2 (CaO mixed with water):
$$\text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca(OH)}_2(aq) + \text{Heat}$$
Step 3 (Lime water exposed to air):
$$\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$$
(b) Types of reaction:
- Step 1: Decomposition reaction (a single compound breaks down into two simpler substances on heating).
- Step 3: Combination reaction (calcium hydroxide and carbon dioxide combine to form a single product, calcium carbonate).
(c) The second step is an exothermic reaction. Calcium oxide reacts vigorously with water to form calcium hydroxide, releasing a large amount of heat — this is why the container becomes hot. It is a combination reaction, and combination reactions typically release energy as two reactants join to form a single, more stable product.
Source: Chapter 1, Section 1.2.1 (Combination Reaction) and Section 1.2 (Decomposition Reaction)
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Explanation
- Examiners expect all three equations balanced with correct state symbols.
- "Decomposition" must be named for Step 1 and "combination" for Step 3 — reversing these loses marks.
- For part (c), link the heat release explicitly to the exothermic nature of combination reactions — this is the reasoning the examiner wants, not just a label. The textbook Activity 1.4 is the direct source for this observation.