What is plaster of Paris? Describe how it is made from gypsum, with the equation and the precaution needed. Write what happens when it is mixed with water, and give three uses.

1. 1 · Define. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O.
2. 2 · Raw material. It is made from gypsum, CaSO₄·2H₂O.
3. 3 · Process and equation. Heat gypsum at about 100 °C (373 K): CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.
4. 4 · Precaution. Do not heat above 100 °C, or all the water is lost and dead burnt plaster forms, which does not set.
5. 5 · Setting. Mixed with water, it sets into hard gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
6. 6 · Uses. Casts for fractured bones, toys and statues, smooth walls and decorative ceilings, blackboard chalk.
7. 7 · Close in one line. So a small change in water of crystallisation turns gypsum into a useful setting powder.

Scoring tip: Write ½H₂O and 1½H₂O carefully; this is where most answers lose marks. Learn both the making and the setting equation.