The difference of water molecules in gypsum and plaster of Paris is (a) \(\frac{5}{2}\) (b) 2 (c) \(\frac{1}{2}\) (d) \(1 \frac{1}{2}\)

Gypsum and Plaster of Paris are two substances that have been used for centuries in building, art, and industry. Gypsum, naturally occurring as a mineral, is commonly found in layered sedimentary deposits. It's composed of calcium sulfate dihydrate, making it particularly useful for making plaster.

Plaster of Paris, on the other hand, derives its name from a large deposit of gypsum located in Paris. It is a quick-setting gypsum plaster consisting of a fine white powder, which hardens when moistened and allowed to dry. This material is used for casting and construction.

Both gypsum and Plaster of Paris have significant historical, cultural, and practical relevance. They're used in restoration works, orthopedics for making casts, in art for sculptures, and in construction for creating supportive walls and ornaments, illustrating their versatility.

The chemical composition of a substance defines its properties and applications. Gypsum, known scientifically as calcium sulfate dihydrate, has the chemical formula \( CaSO4\cdot2H2O \). This indicates that each molecule of gypsum contains one calcium (Ca) atom, one sulfur (S) atom, four oxygen (O) atoms, and is bonded to two water (H2O) molecules.

The presence of water in its structure makes gypsum soft and malleable, which is why it's a preferred material for various construction and artistic applications. It can be ground into a fine powder, mixed with water, and then molded into desired shapes. As it sets, it reverts to the hard, yet brittle, form of gypsum, making it an excellent plaster material.

Plaster of Paris, also known as calcium sulfate hemihydrate, carries the chemical formula \( CaSO4\cdot0.5H2O \). This formula reveals that in Plaster of Paris, for every molecule of calcium sulfate, there is half a molecule of water, making it a 'hemihydrate' – 'hemi' meaning half and 'hydrate' relating to water.

The reduced water content compared to gypsum makes Plaster of Paris much quicker to set, a property exploited in its use. When mixed with water, it re-forms into gypsum but in a much more solid and dense form after drying. This quick-drying property makes it ideal for casts as it can conform quickly to the shape of a limb or for molds in art and architecture because it captures details precisely and sets rapidly.