Plaster of Paris (POP) should be stored in a moisture-proof container because it is highly hygroscopic and absorbs moisture from the air. When POP comes into contact with water or moisture, it undergoes a chemical reaction to form gypsum, a hard solid substance. This reaction can be represented as:
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⋅21H2O+2H2O→CaSO4⋅2H2O
Here, calcium sulfate hemihydrate (POP) reacts with water to form calcium sulfate dihydrate (gypsum), which hardens and cannot be reverted to its original powder form. If POP is not stored properly, it will absorb moisture from the environment, harden prematurely, and become unusable for molding or casting purposes. Hence, a moisture-proof container prevents this unwanted hydration, preserving the powder's quality and usability for later use.
