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) \(\mathrm{CaSO}_{4} \cdot 1 / 2 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CaSO}_{4}\)

Step-by-Step Solution

Answer

Upon heating, gypsum becomes \( \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} \), or plaster of Paris.

Step 1: Understanding Gypsum's Chemical Formula

Gypsum is a mineral composed of calcium sulfate dihydrate, with the chemical formula \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \). The notation indicates that gypsum consists of calcium sulfate molecules each with two water molecules of crystallization.

Step 2: Heating Effect on Gypsum

When gypsum is heated, it loses water molecules and becomes partially dehydrated. This process transforms gypsum into a substance known as plaster of Paris, which is represented chemically as \( \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} \).

Step 3: Evaluating Given Options

Among the provided options, option (c), \( \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} \), matches the partially dehydrated form of gypsum produced upon heating, which is known as plaster of Paris.

Key Concepts

Calcium Sulfate Dihydrate

Calcium sulfate dihydrate, commonly known as gypsum, is a naturally occurring mineral. Its chemical formula is \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \). This indicates that each molecule of calcium sulfate is linked with two water molecules, forming what is known as water of crystallization. This locked-in water is crucial in maintaining the crystalline structure of gypsum.

• Can be mined and occurs as a solid white powder.
• It is widely used in products like drywall and fertilizers.
• The water content makes the mineral soft, allowing for various applications.

This structure also causes gypsum to expand slightly as it sets, which is particularly useful in construction materials allowing for tight fittings and effective fire resistance.

Plaster of Paris

Plaster of Paris is a well-known derivative of gypsum. It has the chemical formula \( \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} \), indicating it is a hemihydrate of calcium sulfate. When gypsum is subjected to higher temperatures, it loses water molecules and partially dehydrates to form plaster of Paris.

• Widely used for casting molds due to its quick-setting nature.
• Commonly utilized in medical casts to immobilize broken bones.
• When mixed with water, it rehydrates and quickly hardens into a solid form.

Its versatility and relatively easy handling make it a staple across various industries, from construction to art.

Chemical Reactions in Heating

When gypsum is heated, a fascinating chemical transformation occurs. The process involves the removal of water molecules, which is a type of reaction known as dehydration. This change can be simplified into a chemical equation which shows the transition from gypsum to plaster of Paris: \[ \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O} \] This reaction takes place typically when gypsum is heated to a temperature around 150 °C (302 °F).

• The loss of water molecules results in a powder form known as plaster of Paris.
• This heating process is reversible; adding water to plaster of Paris will return it to a hard, hydrated form.
• Such transformations involve energy changes and are common in other mineral dehydration processes as well.

These reactions are fundamental in various applications, demonstrating how heat can alter the physical and chemical properties of materials.