Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

Gypsum on heating gives: (a) \(\mathrm{CaS}+\mathrm{O}_{2}\) (b) \(\mathrm{CaO}+\mathrm{SO}_{3}\) (c) \(\mathrm{CaSO}_{4}^{.} 1 / 2 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CaSO}_{4}\)

Step-by-Step Solution

Answer

The correct answer is (c) \( \mathrm{CaSO}_{4}^{.} 1 / 2 \mathrm{H}_2 \mathrm{O} \).

Step 1: Understanding the Problem

We need to understand what happens when gypsum is heated. Gypsum is chemically known as calcium sulfate dihydrate, represented by the formula \( \mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O} \). When it is heated, it undergoes partial dehydration.

Step 2: Dehydration of Gypsum

When gypsum is heated at temperatures around 100-180°C, it loses some of its water of crystallization, transforming into a hemihydrate form. The reaction is: \( \mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CaSO}_{4}^{.} 1 / 2 \mathrm{H}_{2} \mathrm{O} + 1.5 \mathrm{H}_2 \mathrm{O} \). This form is commonly called plaster of Paris.

Step 3: Selecting the Correct Option

From the dehydration reaction, we see that the product formed is the hemihydrate form of calcium sulfate, which is \( \mathrm{CaSO}_{4}^{.} 1 / 2 \mathrm{H}_2 \mathrm{O} \). This corresponds to option (c) in the given choices.

Key Concepts

Understanding Calcium Sulfate Dihydrate

Calcium sulfate dihydrate is the chemical name for gypsum, a naturally occurring mineral widely used in construction and manufacturing. Its chemical formula is \( \mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O} \), which indicates that it contains two water molecules associated with each molecule of calcium sulfate.
These water molecules are known as the "water of crystallization" and are integrated into the mineral's crystalline structure.
When gypsum is heated, these water molecules are released, causing a transformation in its physical and chemical properties.
By understanding the composition of calcium sulfate dihydrate, one can grasp why it behaves the way it does under heating. It becomes a critical point for applications where moisture control or specific physical characteristics are required in construction and art.

Formation of Plaster of Paris

Plaster of Paris is a widely used material that results from heating gypsum to about 100-180°C, causing it to lose some of its water content through dehydration. When this happens, calcium sulfate dihydrate turns into "calcium sulfate hemihydrate" with the formula \( \mathrm{CaSO}_{4}^{.} 1 / 2 \mathrm{H}_2 \mathrm{O} \).
This process results in the hemihydrate commonly referred to as Plaster of Paris, named after its abundant sources near Paris, France.

• Plaster of Paris sets quickly upon adding water, forming a paste that hardens due to the rehydration.
• Ideal for casting molds, construction, and decorative work due to its ease of use and quick setting nature.
• It is important in various industries, from medical (as in making casts) to the arts for gaining precise details in sculptures.

The ability to control the setting time by managing water content and temperature highlights why this material is so widely adopted.

Chemical Reactions in Gypsum Dehydration

The transformation of gypsum (calcium sulfate dihydrate) into Plaster of Paris involves a chemical reaction known as dehydration. This occurs when gypsum is heated, losing water molecules that are part of its crystalline structure. The reaction can be expressed as:
\[ \mathrm{CaSO}_4 \cdot 2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CaSO}_{4}^{.} 1 / 2 \mathrm{H}_2 \mathrm{O} + 1.5 \mathrm{H}_2 \mathrm{O} \]
Understanding this chemical reaction is crucial:

• It involves physical and chemical changes, impacting the properties and applications of the resulting materials.
• Careful control of the reaction conditions (temperature and time) ensures the desired properties of Plaster of Paris.
• Recognizing the reversible nature of this reaction is important for the practical applications, where rehydration can return Plaster of Paris to a hardened state.

This chemical reaction is not just a conversion process; it plays a pivotal role in various industries, making gypsum dehydration a fundamental concept in chemistry and material sciences.