When Plaster of Paris (CaSO₄·½H₂O) is mixed with water, it becomes hard. Which process is responsible for this hardening?
Rehydration to form gypsum
Plaster of Paris is calcium sulphate hemihydrate, CaSO4·½H2O. When it is mixed with water, each formula unit of the hemihydrate picks up the extra water it needs to become the fully hydrated salt calcium sulphate dihydrate, CaSO4·2H2O, which is the mineral gypsum. This rehydration produces interlocking crystals of gypsum that grow into one another and set the paste into a hard, rigid mass.
The reaction is not a neutralisation, because no acid and base are involved; it is not an oxidation, because the oxidation state of calcium (+2) does not change; and it is certainly not a dehydration, because water is being added, not removed. Only the rehydration to gypsum matches the observed hardening.
Hence, the answer is Rehydration to form gypsum.
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