Chapter 2 · Question 15

How is Plaster of Paris prepared from gypsum? Write the chemical equation for its setting when mixed with water. Why should Plaster of Paris be stored in a moisture-proof container?

Q15

How is Plaster of Paris prepared from gypsum? Write the chemical equation for its setting when mixed with water. Why should Plaster of Paris be stored in a moisture-proof container?

Answer Revealed

Direct Answer:

Plaster of Paris is prepared by heating gypsum (

\text{CaSO}_4 \cdot 2\text{H}_2\text{O}

) at 373 K:

\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O}

. Gypsum loses water molecules to form calcium sulphate hemihydrate (Plaster of Paris). When mixed with water, it rehydrates and sets into a hard solid mass of gypsum:

\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}

. Plaster of Paris must be stored in a moisture-proof container because it readily absorbs moisture from the air and sets into a hard lump, becoming unusable.

Simple Explanation

Gypsum is a soft mineral with two water molecules attached. Heat it to 373 K (about 100°C) and it loses one and a half of those water molecules, becoming Plaster of Paris — a fine white powder. The formula looks odd with half a water molecule (

\frac{1}{2}\text{H}_2\text{O}

), but that is because two formula units share one water molecule. When you mix this powder with water, it drinks the water back and hardens into solid gypsum — this is why it is used for plastering broken bones and making casts. Store it airtight, because even humidity from the air can make it set prematurely into a useless lump.

Exam-Ready Structure

Plaster of Paris is an important calcium-based compound whose preparation, setting, and storage relate directly to the concept of water of crystallisation. 1. Preparation from gypsum: Gypsum is calcium sulphate dihydrate with the formula

\text{CaSO}_4 \cdot 2\text{H}_2\text{O}

. When gypsum is heated at 373 K, it loses water molecules and forms calcium sulphate hemihydrate:

\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O}

. The product is Plaster of Paris, a white powder. 2. The half water molecule: The formula

\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}

means that two formula units of

\text{CaSO}_4

share one molecule of water of crystallisation. It is also written as

(\text{CaSO}_4)_2 \cdot \text{H}_2\text{O}

. 3. Setting (hydration): When Plaster of Paris is mixed with water, it rehydrates to form gypsum, setting into a hard solid mass:

\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}

(Plaster of Paris + water

\rightarrow

Gypsum). This property is exploited for its various applications — the paste can be moulded into any shape before it sets. 4. Uses: (a) Doctors use it as plaster for supporting fractured bones in the correct position. (b) Making toys, decorative materials, and statues. (c) Making surfaces smooth. 5. Storage precaution: Plaster of Paris must be stored in a moisture-proof (airtight) container. Reason: It has a strong tendency to absorb moisture from the atmosphere. If exposed to humid air, it will react with the moisture, set into a hard lump of gypsum, and become unusable. 6. The preparation and setting together illustrate a reversible hydration-dehydration cycle: Gypsum

\rightleftharpoons

Plaster of Paris + Water.

Key Points

Plaster of Paris preparation: $\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O}$ (gypsum heated at 373 K)
Plaster of Paris = $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$ (calcium sulphate hemihydrate); gypsum = $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$
Setting reaction: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$
Uses: supporting fractured bones, making toys/decorative items, smoothing surfaces
Storage: moisture-proof container — otherwise it absorbs moisture from air and sets prematurely

Common Mistakes

Writing the formula of Plaster of Paris as $\text{CaSO}_4$ (anhydrous) — it contains half a water molecule per formula unit
Forgetting that heating must be at 373 K, not arbitrary heating — higher temperatures can produce anhydrous $\text{CaSO}_4$ which does not set properly

How is Plaster of Paris prepared from gypsum? Write the chemical equation for its setting when mixed with water. Why should Plaster of Paris be stored in a moisture-proof container?