Plaster of Paris and Water of Crystallization

Preparation — controlled dehydration of gypsum. Gypsum is heated at a mild, controlled temperature (about 373 K, i.e. around 100 degrees Celsius, in NCERT's commonly quoted 330–373 K range). Only part of its water of crystallisation is driven off:

where: $\text{CaSO}_4\cdot2\text{H}_2\text{O}$ is gypsum, $\text{CaSO}_4\cdot\tfrac12\text{H}_2\text{O}$ is Plaster of Paris, and $\Delta$ marks the mild, controlled heat applied — heating strongly for longer drives off the remaining water too, giving fully anhydrous $\text{CaSO}_4$ ("dead burnt plaster"), which will not set properly with water.

Setting — rehydration back to gypsum. Mixing Plaster of Paris with water is the exact reverse reaction, reabsorbing the lost water and hardening into gypsum within minutes:

This setting reaction, together with a slight expansion as it hardens, is why Plaster of Paris fills mould crevices sharply and is prized for casts and models.

Uses and storage. The setting reaction above is exactly why Plaster of Paris is useful in practice: doctors use it to make plaster casts that immobilise fractured bones, it is moulded into decorative models and sculptures, and it is used for smooth surfaces in false ceilings. Because it must stay in its dry, unreacted powder form until deliberately mixed with water, it has to be stored in an airtight, moisture-free container — if left open to humid air, atmospheric moisture will slowly trigger the same setting reaction and turn the powder into a useless hardened lump before it is ever used.

Water of crystallisation, seen twice. Both reactions turn on the same idea: a salt's formula can include a fixed, definite number of water molecules bound into its crystal structure — not loose surface moisture. Washing soda ($\text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O}$) gains ten water molecules when it recrystallises from solution; Plaster of Paris loses some of its water when gypsum is carefully heated. Same concept, opposite direction.

Question: A student heats $344\ \text{g}$ of gypsum, $\text{CaSO}_4\cdot2\text{H}_2\text{O}$, at the controlled temperature until it is fully converted to Plaster of Paris. Find the mass of water driven off and the mass of Plaster of Paris obtained. Also state what would happen instead if the same sample were heated strongly for a long time.

Step 1. Note that $344\ \text{g}$ is exactly two moles of gypsum, since the molar mass of gypsum is $172\ \text{g/mol}$ (found above) and $344 / 172 = 2$.

Step 2. Scale the per-mole water loss found in the derivation. One mole of gypsum loses $27\ \text{g}$ of water, so two moles lose $2 \times 27 = 54\ \text{g}$ of water.

Step 3. Find the mass of Plaster of Paris by mass conservation. Mass of Plaster of Paris $= 344 - 54 = 290\ \text{g}$, which also checks against the molar mass route: two moles of Plaster of Paris at $145\ \text{g/mol}$ gives $2 \times 145 = 290\ \text{g}$.

Step 4. Answer the "what if heated strongly" part. If heated strongly for a long time instead of at the controlled 373 K, all the water of crystallisation would be lost, not just the $54\ \text{g}$ — the product would be fully anhydrous $\text{CaSO}_4$ ("dead burnt plaster"), which does not set properly when later mixed with water and would be useless for casts or moulds. Answer: 54 g of water is lost and 290 g of Plaster of Paris is obtained; strong prolonged heating instead gives useless dead burnt plaster.

• Writing the formula as $\text{CaSO}_4\cdot\text{H}_2\text{O}$ or $\text{CaSO}_4\cdot2\text{H}_2\text{O}$ instead of the correct $\text{CaSO}_4\cdot\tfrac12\text{H}_2\text{O}$ — the half-water-molecule notation is exactly what distinguishes Plaster of Paris from gypsum.
• Forgetting that heating must be controlled and mild — prolonged strong heating gives dead burnt plaster, which will not set with water, a frequently tested distinction.
• Confusing which reaction is "preparation" (heating gypsum, losing water) with which is "setting" (mixing with water, gaining water back) — they are exact reverses of each other.
• Assuming Plaster of Paris is stable in open, humid air — it must be stored airtight, or atmospheric moisture will set it prematurely into a useless hardened lump.

Ask the tutor: Want to try more mass-of-water-lost calculations for different starting masses of gypsum, or compare gypsum, Plaster of Paris and dead burnt plaster side by side?