a thick coating of a paste of Plaster of  Paris (POP) is applied over the bandage on a fractured bone. it becomes hard on drying to keep the fractured bone immobilised. can the change POP be reversed? why?

When water is added to plaster of Paris (POP), it changes to another substance and on drying it hardens. Once the POP has hardened, its shape cannot be changed. Therefore, the change in POP cannot be reversed.