Which of the following is known as 'Plaster of Paris'?

2CaSO₄.H₂O

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

Plaster of Paris is a white powder that, when mixed with water, forms a hard and durable solid. The chemical formula for Plaster of Paris is \(2\text{CaSO}_4 \cdot H_2O\), which indicates that it is a hemihydrate of calcium sulfate. Here's an explanation:

1. Calcium Sulfate (CaSO₄):  Plaster of Paris is derived from calcium sulfate, a compound that naturally occurs as the mineral gypsum (\(CaSO₄ \cdot 2H₂O\)). Gypsum is dehydrated to form Plaster of Paris through a specific heating process.

2. Dehydration: When gypsum is heated to a certain temperature, it loses a portion of its water molecules and undergoes partial dehydration. The chemical reaction involved is:

\[\text{CaSO}_4 \cdot 2H_2O \overset{{\text{Heat}}}{\longrightarrow} 2\text{CaSO}_4 \cdot H_2O + 3H_2O \]

This results in the formation of Plaster of Paris (\(2\text{CaSO}_4 \cdot H_2O\)) along with the release of water.

3. Hemihydrate Structure: The formula \(2\text{CaSO}_4 \cdot H_2O\) indicates that each formula unit of Plaster of Paris contains two calcium sulfate molecules and one water molecule. The water molecule is loosely bound, and this hemihydrate structure is responsible for the ability of Plaster of Paris to rehydrate when mixed with water.

4. Setting and Hardening: When Plaster of Paris is mixed with water, it undergoes a process of rehydration. During this process, the hemihydrate form of calcium sulfate absorbs water and forms a solid mass. The mixture hardens relatively quickly, making Plaster of Paris an excellent material for casting, molding, and creating rigid structures.

Plaster of Paris is widely used in construction, arts and crafts, and medical applications, particularly for casting molds and setting fractured bones. Its ability to set quickly and form a strong, smooth surface makes it a versatile material in various fields.