Which is the product that is formed after the heating of gypsum at 393 K and evaporation of 75% of water content from it?
Try it first: pick an option, then read the explanation.
Understanding the Calcination of Gypsum
Gypsum is a naturally occurring mineral with the chemical formula CaSO₄·2H₂O (Calcium Sulphate Dihydrate). It contains two molecules of water of crystallization. When gypsum is heated carefully under controlled temperatures, it undergoes a process called calcination, where it loses some of this water. This process changes its chemical structure and properties, creating different products used in construction.
Detailed Analysis of the Options
C. Plaster of Paris
This is the correct answer. When gypsum (CaSO₄·2H₂O) is heated to about 393 K (120°C), it loses one and a half molecules of water. This is 1.5 out of 2 total water molecules, which corresponds to a 75% loss of water content. The resulting product is Calcium Sulphate Hemihydrate (CaSO₄·½H₂O), which is commonly known as Plaster of Paris.
The chemical reaction is:
$$ \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{393 \text{K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} $$
A. Cement & B. Geo-polymer
These are incorrect. Cement and geo-polymers are complex binders produced through entirely different processes involving different raw materials (like limestone, clay, silica, and alumina) at much higher temperatures.
D. Calcined lime
This is incorrect. Calcined lime (quicklime) is produced by heating limestone (calcium carbonate, CaCO₃) to drive off carbon dioxide, not by heating gypsum.
Summary: Products from Heating Gypsum
| Product | Formula | Formation Process |
|---|---|---|
| Gypsum | CaSO₄·2H₂O | Natural mineral (raw material). |
| Plaster of Paris | CaSO₄·½H₂O | Heat gypsum to ~393 K (120°C). Loses 75% water. |
| Anhydrite (Dead Burnt Plaster) | CaSO₄ | Heat gypsum above 473 K (200°C). Loses 100% water. |
Study Tips
- Gypsum has 2 waters: Start by remembering the formula for gypsum is CaSO₄·2H₂O.
- Plaster has ½ water: Plaster of Paris is the hemihydrate (hemi = half), so its formula is CaSO₄·½H₂O.
- Calculate the Loss: The water lost is 2 - ½ = 1½. The percentage loss is (1.5 / 2) * 100 = 75%. This directly matches the question.
- Temperature is Key: The temperature of ~393 K (or 120-150°C) is specific to the production of Plaster of Paris. Higher temperatures create "dead burnt" plaster that won't set easily.