The equation to show the reaction between plaster of Paris and water is:

CaSO4⋅12H2O+2H2O→CaSO4⋅2H2O\text{CaSO}_4\cdot \frac{1}{2}\text{H}_2\text{O}+2\text{H}_2\text{O}\rightarrow \text{CaSO}_4\cdot 2\text{H}_2\text{O}CaSO4​⋅21​H2​O+2H2​O→CaSO4​⋅2H2​O

This represents the chemical reaction where calcium sulfate hemihydrate (plaster of Paris) reacts with water to form calcium sulfate dihydrate (gypsum), which hardens as the product.