Generated by claude-sonnet-4-6 · 2026-06-14 10:27 · grounding rag
Model Answer
(i) When blue copper sulphate crystals (hydrated) are heated, they lose water of crystallisation and turn white:
$$\text{CuSO}_4 \cdot 5\text{H}_2\text{O}(s) \xrightarrow{\Delta} \text{CuSO}_4(s) + 5\text{H}_2\text{O}(g)$$
(ii) When sodium hydrogen carbonate is heated during cooking:
$$2\text{NaHCO}_3(s) \xrightarrow{\Delta} \text{Na}_2\text{CO}_3(s) + \text{H}_2\text{O}(g) + \text{CO}_2(g)$$
Source: Chapter 1, Chemical Reactions and Equations
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Explanation
- (i) The key point is that hydrated copper sulphate (blue) loses 5 water molecules on heating to give anhydrous copper sulphate (white). Examiners expect the formula $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ on the LHS.
- (ii) This is a thermal decomposition reaction. The coefficient 2 before NaHCO₃ is essential for balancing. This reaction releases CO₂, which makes dough/batter rise during cooking (baking soda).
- Always include the heat symbol ($\Delta$) above the arrow since energy is supplied.