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?

Answer

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