Question

why the plaster of paris are stored in air tight packs or non wet places [dry places]

Answer 1

it is because plaster of paris forms gypsum on addition with water molecules and becomes hard.........so plaster of paris is stored in the dry places and not wet places..

Answer 2

Plaster of paris can easily absorb moisture to form gypsum which is a white porous material that is hard.Thus in order to avoid this we need to store it in a dry place.