What happens when carbon dioxide is passed in excess through lime water? Write the chemical equation for the reaction.
Generated by claude-sonnet-4-6 · 2026-06-17 12:36 · grounding rag
Model Answer
When CO₂ is passed in excess through lime water, the white milky precipitate of calcium carbonate dissolves to form soluble calcium bicarbonate, and the solution turns colourless again.
$$\text{CaCO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \rightarrow \text{Ca(HCO}_3)_2(aq)$$
Source: Chapter 1, Section 1.2.1 (and general chemistry knowledge consistent with the passage)
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Explanation
- The first reaction (lime water turning milky) is: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O — this is the standard test for CO₂.
- The excess CO₂ reaction converts insoluble CaCO₃ into soluble Ca(HCO₃)₂, clearing the solution. Examiners expect both the observation and the balanced equation.
- Note that the source passage mentions CaCO₃ formation from Ca(OH)₂ + CO₂ (equation 1.14); the excess CO₂ step is the standard follow-up reaction tested in board exams.