Gypsum, heat and a plaster cast
Half a water molecule? Two units share one.
A cast for a broken arm
A doctor mixes a white powder with water and wraps it round a broken arm.
What happens to the wet paste in a few minutes?
What this lesson covers
The idea
Heating gypsum (CaSO4.2H2O) at 373 K gives Plaster of Paris, CaSO4.½H2O, in which two formula units share one water molecule; mixed with water it changes back to gypsum as a hard solid mass.
A cast for a broken arm
A doctor mixes a white powder with water and wraps it round a broken arm.
What happens to the wet paste in a few minutes?
- It sets into a hard solid that holds the bone still
- It dissolves and runs away
- It bubbles and gives off a gas
Heat gypsum, then add water
Two CaSO₄ units are shown with their water molecules. Press the buttons.
A molecule cannot be cut in half. Two CaSO₄ units share one water molecule, so each unit has ½ of it. That is why it is written CaSO₄·½H₂O.
Gypsum ⇄ Plaster of Paris
Heating gypsum (CaSO₄·2H₂O) at 373 K gives Plaster of Paris, CaSO₄·½H₂O, in which two formula units share one water molecule. Mixed with water it changes back to gypsum as a hard solid mass.
Plaster of Paris is a white powder. It is used for plaster to support fractured bones, for toys, for decoration, and for making surfaces smooth.
- Substance | Formula | Water per CaSO₄ unit
- Gypsum | CaSO₄·2H₂O | 2
- Plaster of Paris | CaSO₄·½H₂O | ½ (shared)
Notes
Heating gypsum (CaSO₄·2H₂O) at 373 K gives Plaster of Paris, CaSO₄·½H₂O, in which two formula units share one water molecule; mixed with water it changes back to gypsum as a hard solid mass.
Check yourself
At what temperature is gypsum heated to make Plaster of Paris?
Answer: 373 K
Gypsum is heated at 373 K (which is 100 °C).
Which is the formula of Plaster of Paris?
Why is only "half" a water molecule written in CaSO₄·½H₂O?
Plaster of Paris is mixed with water. It changes back to …
- CaSO₄·2H₂O. That is gypsum, before heating.
- CaSO₄. That has no water of crystallisation at all. Plaster of Paris keeps ½ H₂O.
- CaSO₄·½H₂O — correct. Yes! Two CaSO₄ units share one water molecule.
- Two CaSO₄ units share one water molecule — correct. Yes! One H₂O for two units is ½ for each.
- Half of the molecule burns away. Water molecules are never cut. Two units share one.
- Water can be split in half like a fruit. A molecule is not split. The water is shared between two units.
- sodium carbonate. No sodium is involved. It goes back to gypsum.
- gypsum, as a hard solid mass — correct. Yes! That is why it makes a cast.
- a clear salt solution. It does not dissolve. It sets as a hard solid.