Science — CBSE Class 10 board question

(i) FeSO₄ crystals exist as hydrated salt — FeSO₄·7H₂O. On heating, the water of crystallisation is driven off, which condenses as tiny droplets on the cooler upper walls of the boiling tube.

(ii) The green colour of ferrous sulphate crystals changes to reddish-brown, as FeSO₄ decomposes to form ferric oxide (Fe₂O₃).

$$2\text{FeSO}_4 \xrightarrow{\Delta} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3$$

(iii) 7 molecules of water are attached per molecule of FeSO₄.

• (I) Copper sulphate: CuSO₄·5H₂O
• (II) Sodium carbonate: Na₂CO₃·10H₂O

(iv) Plaster of Paris is obtained by heating gypsum (CaSO₄·2H₂O) at 373 K:

$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O}$$

Uses: (1) Making casts for fractured bones. (2) Making statues and decorative materials.

Source: Chapter 1, Section 1.2.2; Chapter 2 (Acids, Bases and Salts)

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• (i) Key phrase: "water of crystallisation" — examiners expect this term.
• (ii) The colour change (green → reddish-brown) is due to formation of Fe₂O₃; mention both colours.
• (iii) The 7 water molecules must be stated clearly. Both crystal formulae carry 1 mark — memorise them accurately (CuSO₄·5H₂O and Na₂CO₃·10H₂O).
• (iv) This sub-question is from the Acids, Bases and Salts chapter (Ch. 2). State the temperature (373 K), write the equation, and give two distinct uses for full marks.