Question

How is plaster of paris obtained ? What reaction is involved in the setting of a paste of plaster of paris? What is it used for? [3 MARKS]

Answer 1

Explanation: 3 Marks

1. Plaster of Paris is chemically different from gypsum in terms of water of crystallisation. Gypsum has 2 moles of water per mole of CaSO₄. It can also be written as if one mole of water of crystallisation is present for two moles of CaSO₄, (2CaSO₄ . $H_{2}O$). Gypsum on heating at 373 K gets converted into Plaster of Paris.

2. ${CaSO}_4.2H_{2}O\to {CaSO}_4.\left(\frac{1}{2}\right)H_{2}O+\frac{3}{2}{H}_{2}O$

Gypsum Plaster or Paris

When Plaster of Paris is mixed with water, it gets converted into gypsum.

${CaSO}_4.\left(\frac{1}{2}\right)H_{2}O+\left(\frac{3}{2}\right)H_{2}O\to {CaSO}_4.2H_{2}O$

Plaster of Paris Gypsum

3. Plaster of paris is used for making statues and for setting of fractured bones by making cast around the fractured limbs.