Plaster of Paris

What is plaster of Paris?

Plaster of Paris is made by heating the mineral gypsum. Large deposits were originally found outside of Paris in France hence its name. When gypsum is heated to about 150°C it losses water and produces the powder, plaster of Paris

Gypsum  =heat=> Plaster of Paris + steam

2CaSO₄·2H₂O =heat=> 2CaSO₄·½H₂O + 3H₂O

When water is added to the plaster of Paris powder it rehydrates (absorbs water) and quickly hardens.

Plaster of Paris + water ==> Gypsum

2CaSO₄·½H₂O + 3H₂O ==> 2CaSO₄·2H₂O + heat

Plaster of Paris can be used for casts to hold broken limbs in place, modeling casts, sculptures and in plasterboard walls and ceilings, commonly called Gyprock.

Large amounts of plaster of Paris placed directly onto the skin can cause serious burns because of the heat produced! 

The chemical formula for plaster of Paris is CaSO₄·½H₂O which means that there are two molecules of water around each CaSO₄ group.