When gypsum is heated to 393 K, it forms :
(a) Dead burnt plaster
(b) CaSO4 . 0.5H2O
(c) CaSO4 . 5H2

Q: Why is it called Plaster of Paris?

It is called Plaster of Paris because large deposits of gypsum, the raw material used to make it, were originally found in Montmartre, near Paris. It was extensively used by Parisian plasterers in the 18th century.

Q: Can we write the formula of Plaster of Paris as $2\text{CaSO}_4 \cdot \text{H}_2\text{O}$?

Yes, absolutely! Writing it as $\text{CaSO}_4 \cdot 0.5\text{H}_2\text{O}$ might seem strange because you can't have half a water molecule. The formula $2\text{CaSO}_4 \cdot \text{H}_2\text{O}$ is actually more accurate, as it shows that two formula units of calcium sulfate share one molecule of water.

Q: What happens if we add water back to Plaster of Paris?

When you mix Plaster of Paris with water, it undergoes an exothermic hydration reaction, taking back the $1.5$ molecules of water it lost to form a hard, solid mass of gypsum again. This is why it is used for making casts and molds.

Q: Why is anhydrous calcium sulfate called 'dead burnt plaster'?

It is called 'dead' because it loses the remarkable property of Plaster of Paris to set into a hard mass when mixed with water. The high heat alters its crystal structure so much that it no longer readily rehydrates.

Q: Is the water loss from gypsum sudden or gradual?

It is a stepwise dehydration process. At $393\text{ K}$, it loses $1.5$ molecules of water to form the hemihydrate. If heated further (above $393\text{ K}$), it loses the remaining water to become anhydrous.

Question

When gypsum is heated to $393	ext{ K}$, it forms :

Options:

(A) Dead burnt plaster

(B) $	ext{CaSO}_4 \cdot 0.5	ext{H}_2	ext{O}$

(C) $	ext{CaSO}_4 \cdot 5	ext{H}_2	ext{O}$

(D) Anhydrous $	ext{CaSO}_4$

Sigma Percentile · Accepted Answer

### The Starting Material: Gypsum

Let's begin by looking at the starting material for our reaction. Gypsum is a naturally occurring, soft sulfate mineral composed of calcium sulfate dihydrate. Its chemical formula is **$	ext{CaSO}_4 \cdot 2	ext{H}_2	ext{O}$**. This means that for every unit of calcium sulfate, there are exactly two molecules of water trapped within its crystal lattice. This water is known as the *water of crystallization*, and it plays a crucial role in the physical properties of the mineral.

### The Critical Temperature: 393 K

Now, what happens when we apply heat to this mineral? Chemistry is often a game of precise conditions. If we carefully heat gypsum to exactly **$393	ext{ K}$** (which is $120^\circ	ext{C}$), a specific thermal decomposition occurs. The heat energy is just enough to break some of the bonds holding the water molecules in the crystal lattice, but not all of them.

At this specific temperature, gypsum loses exactly one and a half molecules of water per formula unit. The chemical equation for this transformation is:

$$	ext{CaSO}_4 \cdot 2	ext{H}_2	ext{O} \xrightarrow{393	ext{ K}} 	ext{CaSO}_4 \cdot 0.5	ext{H}_2	ext{O} + 1.5	ext{H}_2	ext{O}$$

The resulting compound, **$	ext{CaSO}_4 \cdot 0.5	ext{H}_2	ext{O}$**, is chemically known as calcium sulfate hemihydrate. However, you probably know it by its much more famous common name: **Plaster of Paris**. It got this name because large deposits of gypsum were originally found near Paris, France, and it was widely used there for making plaster casts and architectural details.

### The Danger of Overheating: Dead Burnt Plaster

There is a very important catch here. Temperature control is absolutely critical. What if we get careless and heat the gypsum beyond $393	ext{ K}$?

If the temperature exceeds this limit, the thermal energy becomes too high, and the remaining half molecule of water is also driven off. The compound loses all its water of crystallization to become completely anhydrous calcium sulfate ($	ext{CaSO}_4$).

This anhydrous form is commonly referred to as **dead burnt plaster**. Why "dead"? Because unlike Plaster of Paris, which readily absorbs water to set into a hard mass, anhydrous calcium sulfate does not easily rehydrate. It has lost its "life" or its ability to set, making it useless for molding or casting.

Therefore, strictly at $393	ext{ K}$, the product is Plaster of Paris, making option (b) the correct answer.

Animated Solution for Chemistry - s and p-Block Elements: When gypsum is heated to $393 K$ , it forms :

Select Answer:

Visualized Solution

$G y p s u m$

$H e a t in g G y p s u m$

$F or ma t i o n o f P l a s t er o f P a r i s$

$O v er h e a t in g$

$C o n c l u s i o n$

The Sigma Insight: Alkaline Metals

The Starting Material

The Critical Temperature: 393 K

The Danger of Overheating

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Animated Solution for Chemistry - s and p-Block Elements: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } When gypsum is heated to 393 K, it forms :