Plaster of Paris should be stored in a moisture-proof container. Explain why?

Plaster of Paris is chemically known as calcium sulfate hemihydrate, which is represented by the formula (CaSO₄·½H₂O). When you hear 'hemihydrate,' it means that for every one molecule of calcium sulfate, there's half a molecule of water chemically bonded within its structure.
This unique formation enables Plaster of Paris to transform when mixed with water. The water rehydrates the plaster back into its original form, gypsum, also known as calcium sulfate dihydrate (CaSO₄·2H₂O).
Understanding this chemical reaction is crucial for appreciating why proper storage is essential for Plaster of Paris. If it prematurely comes into contact with moisture, the chemistry changes, and so does its usefulness.

Plaster of Paris has a high reactivity with water. This means that it easily absorbs moisture from its surroundings. When mixed with water, Plaster of Paris undergoes an exothermic reaction—heat is released as the mixture changes from calcium sulfate hemihydrate back to calcium sulfate dihydrate (gypsum).
During this process, it hardens and sets quickly. This rapid reaction is what makes Plaster of Paris perfect for casting molds, sculptures, and quick repairs. However, this same property can cause it to harden prematurely if exposed to moisture in the air.
If not stored properly, moisture can seep into the plaster and trigger this reaction even before its intended use, rendering it unusable. Ensuring it remains dry preserves its effectiveness.

To maintain the integrity of Plaster of Paris, it must be stored in a moisture-proof container. Such containers are designed to block out humidity and prevent any water vapor from entering.
Some useful options might include airtight plastic bags, sealed plastic containers, or even vacuum-sealed packaging. The goal is to have an environment where no moisture can disrupt the finely powdered material.
This kind of storage is essential because even a small amount of moisture can start the hardening process unwantedly. Keeping Plaster of Paris in a moisture-proof container ensures it stays in its best, usable form until you are ready to mix it with water intentionally.
Always check the storage conditions, and make sure to reseal the container immediately after use to protect the remaining plaster.