Plaster of Paris should be stored in a moisture-proof container. Explain why?
Plaster of Paris should be stored in a moisture-proof container because it reacts with moisture (water) to form gypsum.
When Plaster of Paris comes in contact with water, it changes to gypsum, giving a hard solid mass. The reaction is:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (Plaster of Paris) (Gypsum)
If stored in an ordinary container, atmospheric moisture would cause Plaster of Paris to gradually convert into gypsum, making it unusable for purposes like making toys, materials for decoration, and for making surfaces smooth.
Explanation
The textbook clearly states that Plaster of Paris is a white powder that changes to gypsum when mixed with water, forming a hard solid mass. Since Plaster of Paris readily reacts with water, it must be protected from atmospheric moisture to prevent its conversion to gypsum. This is a direct application of the chemical property described in the text.