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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.