Gypsum: Key Ingredient for Calcium Sulphate Hemihydrate

The question asks to identify the substance used in the preparation of calcium sulphate hemihydrate. This compound is commonly known as Plaster of Paris.

Understanding Calcium Sulphate Hemihydrate

Calcium sulphate hemihydrate has the chemical formula \(\text{CaSO}_4·\frac{1}{2}\text{H}_2\text{O}\). It is formed when gypsum is heated.

Preparation Using Gypsum

The process involves heating gypsum, which is calcium sulphate dihydrate (\(\text{CaSO}_4·2\text{H}_2\text{O}\)), under controlled conditions. Typically, this heating occurs at temperatures between 120°C and 180°C.

Heating drives off about half of the water molecules associated with each formula unit of calcium sulphate.

The chemical equation representing this transformation is:

\( \text{CaSO}_4·2\text{H}_2\text{O} \xrightarrow{\Delta} \text{CaSO}_4·\frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \)

In this reaction:

• Gypsum (\(\text{CaSO}_4·2\text{H}_2\text{O}\)) is the starting material.
• Calcium Sulphate Hemihydrate (\(\text{CaSO}_4·\frac{1}{2}\text{H}_2\text{O}\)) is the product, also known as Plaster of Paris.
• The \(1\frac{1}{2}\text{H}_2\text{O}\) indicates the water released as steam during the heating process.

Evaluating the Options

Let's examine the provided options to see why Gypsum is the correct choice:

• Sodium Hydrogen Carbonate (\(\text{NaHCO}_3\)): Commonly called baking soda, this is an alkaline salt used primarily in cooking and cleaning. It is not involved in making calcium sulphate hemihydrate.
• Copper Sulphate (\(\text{CuSO}_4\)): Often found as copper(II) sulphate pentahydrate (\(\text{CuSO}_4·5\text{H}_2\text{O}\)), it is a distinct chemical compound used in agriculture and industry, unrelated to the preparation of calcium sulphate hemihydrate.
• Sodium Carbonate (\(\text{Na}_2\text{CO}_3\)): Known as washing soda, this is used for laundry and cleaning. It does not serve as a precursor for calcium sulphate hemihydrate.
• Gypsum (\(\text{CaSO}_4·2\text{H}_2\text{O}\)): This is the hydrated form of calcium sulphate. As demonstrated by the chemical reaction, heating gypsum is the standard method for producing calcium sulphate hemihydrate (Plaster of Paris).

Final Determination

Based on the chemical process, Gypsum is the essential raw material required for the preparation of calcium sulphate hemihydrate.