Question

Why does plaster of Paris set to a hard mass after nixing with water and leaving it for sometimes .

Answer 1

this is because of large amount of water

Answer 2

1)Plaster of paris is dry powder (molecules are apart)
2)When water is mixed it becomes hydrated
3)But it absorbs the water over time
4) In the process the water acts as an adhesive and it binds the molecules.(point 1)
5) This results in a hard mass