Science — AI-generated practice question

Observation 1: When CO₂ is passed through lime water, it turns milky (white precipitate forms). This is due to the formation of insoluble calcium carbonate.

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

Observation 2: When excess CO₂ is passed, the milky solution turns clear/colourless again. This happens because calcium carbonate reacts with excess CO₂ and water to form soluble calcium bicarbonate.

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

Source: Chapter 1, Section 1.2.1; Chapter 5, Section 5.3

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• Examiners expect two separate observations clearly stated — milky, then clear again.
• Both balanced equations are required; missing either costs marks.
• The key reasoning (CaCO₃ is insoluble; Ca(HCO₃)₂ is soluble) may earn an extra mark if mentioned briefly.
• The second equation is not directly in the source passages but is standard CBSE curriculum knowledge linked to Activity 5.4 context; the first equation is explicitly given as equation 1.14 in Chapter 1.