Science (086) — AI-generated practice question

When CO₂ is passed through lime water, it reacts with calcium hydroxide Ca(OH)₂ to form a white precipitate of calcium carbonate (CaCO₃):

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

When excess CO₂ is passed, the insoluble CaCO₃ reacts with CO₂ and water to form calcium bicarbonate Ca(HCO₃)₂, which is soluble in water, causing the precipitate to dissolve:

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

Source: Chapter 1, Section 1.2.1 (Combination Reaction)

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• Examiners expect both equations — formation of CaCO₃ and its conversion to soluble Ca(HCO₃)₂.
• Key term: calcium bicarbonate (soluble) — this is why the precipitate disappears.
• The second equation is the key chemical change being asked about; don't omit it.
• State symbols (s) and (aq) add value but are not strictly penalised if missing in short answers.