Question

Write an equation to show the reaction between Plaster of Paris and water?
A. $\underset{Plaster\,of\,Paris}{\mathop{CaS{{O}_{4}}\cdot \dfrac{1}{2}{{H}_{2}}O}}\,+\underset{Water}{\mathop{\dfrac{3}{2}{{H}_{2}}O}}\,\to \underset{Gypsum}{\mathop{CaS{{O}_{4}}.2{{H}_{2}}O}}\,$
B. $\underset{Plaster\,of\,Paris}{\mathop{CaS{{O}_{4}}\cdot {{H}_{2}}O}}\,+\underset{Water}{\mathop{\dfrac{3}{2}{{H}_{2}}O}}\,\to \underset{Gypsum}{\mathop{CaS{{O}_{4}}.\dfrac{5}{2}{{H}_{2}}O}}\,$
C. $\underset{Plaster\,of\,Paris}{\mathop{CaS{{O}_{4}}\cdot {{H}_{2}}O}}\,+\underset{Water}{\mathop{{{H}_{2}}O}}\,\to \underset{Gypsum}{\mathop{CaS{{O}_{4}}.2{{H}_{2}}O}}\,$
D. $\underset{Plaster\,of\,Paris}{\mathop{CaS{{O}_{4}}\cdot \dfrac{1}{2}{{H}_{2}}O}}\,+\dfrac{1}{2}\underset{Water}{\mathop{{{H}_{2}}O}}\,\to \underset{Gypsum}{\mathop{CaS{{O}_{4}}.2{{H}_{2}}O}}\,$

Answer 1

Hint: Heating of gypsum at a particular temperature is responsible for the production of plaster of Paris. When the plaster of Paris reacted with water, it again converted into gypsum.

Complete Solution :
When we add water to the plaster of Paris or when the plaster of Paris reacts with water it liberates heat through crystallization because it is an exothermic reaction. Gypsum is produced by heating plaster of Paris to about 150 - degree Celsius.
The chemical formula of the gypsum is $CaS{{O}_{4}}\cdot 2{{H}_{2}}O$ and the Plaster of Paris is also known as the Calcium Sulphate hemihydrates with the chemical formula$CaS{{O}_{4}}\cdot \dfrac{1}{2}{{H}_{2}}O$
Therefore the balanced chemical reaction between the plaster of Paris and water is
$\underset{Plaster\,of\,Paris}{\mathop{CaS{{O}_{4}}\cdot \dfrac{1}{2}{{H}_{2}}O}}\,+\underset{Water}{\mathop{\dfrac{3}{2}{{H}_{2}}O}}\,\to \underset{Gypsum}{\mathop{CaS{{O}_{4}}.2{{H}_{2}}O}}\,$
 Hence option (A) is the correct option.

The chemical formula of the gypsum is $CaS{{O}_{4}}\cdot 2{{H}_{2}}O$ commonly known as calcium sulphate dihydrate In option (B) the gypsum is not a hydrate that is not having two water molecules and hence this option is not the correct option.
Plaster of Paris is also known as the Calcium Sulphate hemihydrates with the chemical formula $CaS{{O}_{4}}\cdot \dfrac{1}{2}{{H}_{2}}O$

- In option (C) the plaster of Paris is not hemi hydrate hence this is also not correct.
The chemical formula of the gypsum is $CaS{{O}_{4}}\cdot 2{{H}_{2}}O$ commonly known as calcium sulphate dihydrate. - In option (D) the gypsum is not a hydrate that is not having two water molecules and hence this option is not the correct option.
So, the correct answer is “Option A”.

Note: When we add water to the plaster of Paris it will reform into gypsum which is also a naturally occurring product whereas plaster of Paris is a manufactured product. Plaster of Paris can be moulded into different shapes when it is moistened, but gypsum does not have that property.