A substance 'X' is used as a building material and is insoluble in water. When it reacts with dil. HCl, it produces a gas which turns lime water milky.
Generated by claude-sonnet-4-6 · 2026-06-15 06:47 · grounding rag
Model Answer
(i) Substance 'X' is Calcium Carbonate.
Chemical formula: CaCO₃
(ii)
Reaction 1 – CaCO₃ is insoluble in water (no equation needed, it is a physical property).
Reaction 2 – CaCO₃ reacts with dilute HCl to produce CO₂ gas:
$$\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$$
Reaction 3 – CO₂ turns lime water milky:
$$\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$$
(White precipitate of CaCO₃ makes lime water milky.)
Source: Chapter 2, Acids, Bases and Salts; Chapter 1, Section 1.2.1
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Explanation
- The clues — building material, insoluble in water, reacts with acid to give CO₂ — all point to CaCO₃ (limestone/marble/chalk, all used in construction).
- Examiners expect both equations: the acid–carbonate reaction AND the lime water reaction. Missing either loses marks.
- Write state symbols — (s), (aq), (g), (l) — as CBSE awards a mark for balanced equations with correct states.
- The lime-water reaction is the same as equation 1.14 from the textbook (reverse context), so knowing it from Chapter 1 is helpful here.