Question

why plaster of pairs is stored in air tight packs or non wet places dry places?

Answer 1

plaster of paris immediately reacts with water and forms gypsum and that is too hard soo plater of paris is stored in dry places

Answer 2

Plaster of Paris has a tendency to absorb moisture and become gypsum which is hard and it can't be used any more. So to prevent absorption of moisture from air, it is stored in tight packs or dry place.