Baking soda, washing soda, plaster of Paris and water of crystallisation sheet3-mark answer frameHow is plaster of Paris prepared? Write the equation. What happens when it is mixed with water? Give two uses.
How to lay out the answer. Follow the frame, then write it in your own words.
- 1 · Define in one line. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O.
- 2 · Preparation. Heat gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.
- 3 · Mixing with water. It sets into a hard mass of gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
- 4 · Uses.
- Plaster for supporting fractured bones.
- Making toys, decorative items and smooth surfaces.
- 5 · Link to real life. Hospital plaster casts and many Ganesh idols are made of plaster of Paris.
- 6 · Close in one line. So losing or gaining water of crystallisation turns gypsum into plaster of Paris and back.
Scoring tip: Write 373 K and the half water molecule. Writing ½H₂O correctly earns the formula mark.
