Science (086) — AI-generated practice question

(a) Sodium Hydroxide — Chlor-Alkali Process:
When electricity is passed through brine (aqueous NaCl), it decomposes. This is called the chlor-alkali process.

$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$

Cl₂ is released at the anode; H₂ at the cathode; NaOH forms near the cathode.

(b) Washing Soda:
Sodium carbonate is recrystallised with water to give washing soda (Na₂CO₃·10H₂O):

$$\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3.10\text{H}_2\text{O}$$

Uses: (i) Used in glass, soap, and paper industries. (ii) Used as a domestic cleaning agent and to remove permanent hardness of water.

(c) Plaster of Paris:
$$\text{CaSO}_4.2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4.\tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\text{H}_2\text{O}$$

When mixed with water, Plaster of Paris rehydrates and converts back to gypsum (CaSO₄·2H₂O), forming a hard solid mass:

$$\text{CaSO}_4.\tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4.2\text{H}_2\text{O}$$

Practical use: Used by doctors to set fractured bones in the correct position.

Source: Chapter 2, Section 2.4.3 and 2.4.4

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• (a) Examiners expect the process name ("chlor-alkali"), the balanced equation, and mention of products at each electrode — all three earn marks.
• (b) The key point is that Na₂CO₃ is recrystallised (not just mixed) with water. Write the equation and give two distinct uses from the textbook list.
• (c) Two equations are needed: formation of Plaster of Paris from gypsum, and its setting with water. The chemical explanation (rehydration → gypsum) and one use (bone-setting/fractured bones) are expected. Don't forget the temperature (373 K) in the heating equation.