Animated Solution for Chemistry - s and p-Block Elements: The number of water molecules in gypsum, dead burnt plaster and plaster of Paris, respectively are

Have you ever wondered why some dry-looking powders suddenly turn into a hard, rock-solid mass when you add water to them? The secret lies in a fascinating chemical phenomenon known as water of crystallization. Many salts, especially those of alkaline earth metals like calcium, have a unique ability to trap water molecules within their crystal lattice. These water molecules aren't just wet puddles; they are chemically bound and dictate the physical properties of the compound.

Today, we are going to explore three very famous compounds of calcium sulphate that are heavily tested in JEE and NEET: Gypsum, Dead Burnt Plaster, and Plaster of Paris.

The Natural Dihydrate

Let's start with the parent compound, Gypsum. It is a naturally occurring mineral that is widely used in fertilizers and as the main constituent in many forms of plaster, blackboard chalk, and drywall.

If we look at its chemical formula, it is written as ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$. This is known as calcium sulphate dihydrate. The 'di' stands for two, meaning that for every one formula unit of calcium sulphate, there are exactly two molecules of water trapped in the crystal structure.

Plaster of Paris

Now, chemistry gets interesting when we apply heat. If we take gypsum and heat it very carefully to exactly $393\text{ K}$ (which is $120^{\circ }\text{C}$), it undergoes a partial dehydration. It doesn't lose all its water, but rather, it loses three-quarters of it.

The reaction looks like this:

The resulting compound, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$, is calcium sulphate hemihydrate, famously known as Plaster of Paris (POP). The name comes from the fact that abundant deposits of gypsum were originally found near Paris. In this compound, the number of water molecules per formula unit is $0.5$.

Dead Burnt Plaster

But what happens if we get a little too enthusiastic with the heating? If we heat gypsum strongly above $393\text{ K}$, the thermal energy becomes too much for the remaining water molecules to hold on. The crystal lattice completely breaks down its hydrogen bonds, and all the water of crystallization is driven off.

The completely anhydrous form, ${\text{CaSO}}_4$, is left behind. This is called Dead Burnt Plaster. It is called 'dead' because, unlike Plaster of Paris, it has lost its ability to set into a hard mass when mixed with water. Since it is completely anhydrous, the number of water molecules is $0$.

Bringing it all together to answer our question: - Gypsum has $2$ water molecules. - Dead Burnt Plaster has $0$ water molecules. - Plaster of Paris has $0.5$ water molecules.

Therefore, the correct sequence is $2,0,\text{ and }0.5$.