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Gypsum is a naturally occurring mineral widely used in construction, agriculture, and industry. Chemically, gypsum is calcium sulfate dihydrate, represented by the formula CaSO₄·2H₂O. This means that each formula unit of gypsum contains one calcium ion (Ca²⁺), one sulfate ion (SO₄²⁻), and two water molecules of crystallization.
The water of crystallization gives gypsum its characteristic properties, including its softness, ability to set when heated, and workability in plaster and cement. Heating gypsum to about 100–150°C partially removes water, forming plaster of Paris (CaSO₄·½H₂O), which is widely used in construction, sculptures, and medical casts. On further heating above 180°C, it converts to anhydrous calcium sulfate (CaSO₄).
Gypsum occurs naturally as crystalline rocks, beds, or as an evaporite mineral. Its chemical composition is important in industry, because the hydration state determines its use. For example, CaSO₄·2H₂O is used directly in construction and soil conditioning, while plaster of Paris is used for making molds and decorative designs.
Other options are incorrect:
CaSO₄ represents anhydrous calcium sulfate, not gypsum.
CaSO₄·½H₂O is plaster of Paris, obtained after partial heating.
CaSO₄·3H₂O does not exist naturally and is not chemically stable.
Understanding gypsum’s formula helps students link mineral chemistry, hydration states, and practical applications. Gypsum is an excellent example of a hydrated salt in inorganic chemistry, illustrating concepts like water of crystallization, heating effects, and industrial significance.
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