The chemical formula of gypsum is _________.

CaSO₄ . 2H₂O

The correct answer is option 4. CaSO₄ . 2H₂O.

Gypsum is a mineral and a common form of hydrated calcium sulfate. Its chemical formula is \( \text{CaSO}_4 \cdot 2H_2O \). Let's break down the components of this chemical formula:

1. Calcium Sulfate (CaSO₄): The main part of the formula is calcium sulfate, which is a compound composed of calcium (Ca), sulfur (S), and oxygen (O). In its anhydrous form (without water molecules), calcium sulfate is represented as \( \text{CaSO}_4 \).

2. Water Molecules (2H₂O): Gypsum is a hydrated form of calcium sulfate, meaning it contains water molecules in its structure. The "·2H₂O" indicates that each formula unit of gypsum is associated with two water molecules.

The chemical formula \( \text{CaSO}_4 \cdot 2H_2O \) signifies that each calcium sulfate unit is accompanied by two molecules of water. The water molecules are often referred to as water of crystallization, and they are an integral part of the crystal structure of gypsum.

When gypsum is heated, it can undergo a process known as dehydration, where the water molecules are driven off, resulting in the formation of the anhydrous form of calcium sulfate, \( \text{CaSO}_4 \). The removal of water causes gypsum to lose its structure and may lead to the formation of a powder known as Plaster of Paris.

In summary, the chemical formula \( \text{CaSO}_4 \cdot 2H_2O \) represents the hydrated form of calcium sulfate, known as gypsum, which is commonly found in nature and has various uses, including in construction, agriculture, and as a setting agent in plaster.