Imagine you have a dry powder that, when you mix it with water, turns into a thick paste. Within minutes, that paste gets warm, then hardens into a solid, rock-like mass. That is exactly what Plaster of Paris does. It is the reason doctors use it to make casts for broken bones — you wrap the wet paste around the limb, it sets hard, and the bone stays immobilised while it heals.
The trick is that the powder is thirsty. It wants to grab water molecules and lock them into its crystal structure. When you give it that water, it transforms into a different, harder substance.
The Chemistry in One Line
Plaster of Paris is calcium sulphate hemihydrate, written as CaSO4⋅21H2O. The "hemi" means half — each formula unit holds only half a water molecule.
It is made by heating gypsum (calcium sulphate dihydrate, CaSO4⋅2H2O) to about 150∘C. The heat drives off three-quarters of the water:
CaSO4⋅2H2OΔCaSO4⋅21H2O+23H2O
When you later add water to the hemihydrate, it greedily takes back that lost water and reverts to gypsum:
CaSO4⋅21H2O+23H2O⟶CaSO4⋅2H2O
This rehydration is what makes it set — the newly formed gypsum crystals interlock into a hard, solid mass.
Watch out
If you heat gypsum above 200∘C, you drive off all the water and get anhydrous calcium sulphate (CaSO4). That powder does not set with water — it is useless for plaster. The temperature must be controlled carefully.
Why "Plaster of Paris"?
The name comes from a large deposit of gypsum found in the Montmartre district of Paris. In the 1700s, it was widely used there for making plaster walls and ceilings. The name stuck.
Key Properties for Exams
Setting time: A few minutes to half an hour, depending on purity and temperature. You can slow it down by adding a little glue or borax.
Volume expansion: It expands slightly on setting — this is why it fills a mould perfectly, capturing fine details. (Contrast with cement, which shrinks.)
Not waterproof: The set gypsum is slightly soluble in water and will soften if soaked.
Important
The formula CaSO4⋅21H2O is the exact composition. Many students mistakenly write CaSO4⋅H2O — that would be the monohydrate, which does not exist in this context. The half-water is crucial.
This hemihydrate, CaSO4.1/2 H2O, is Plaster of Paris. When mixed with water it sets into a hard mass by re-forming gypsum, which is why it is used for making moulds, casts, and in setting fractured bones.