Question

(a) What are the common names of (a) CaOCl₂, and (b) Na₂CO₃.10H₂O?
(b) Why should plaster of Paris be stored in a moisture-proof container?
​(c) Explain why, while diluting a concentrated acid, acid should be added to water and not water to the acid.

Answer 1

(a) CaOCl₂ is chemically known as calcium oxychloride and common name is bleaching powder.
Na₂CO₃.10H₂O is chemically known as sodium carbonate decahydrate and its common name is washing soda.

(b) Plaster of Paris should be stored in a moisture-proof container because plaster of Paris, a powdery mass, absorbs water to form a hard solid known as gypsum.

${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}+1\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$

(c) The dissociation of acid is a highly exothermic reaction. When acid is slowly added to water, the heat produced is absorbed by the water. But if the water is added to the acid, the high amount of heat produced may lead to splashing of water. Thus, when diluting, the acid is always added to the water.