Question

Temporary hardness may be removed from water by adding:
A: \[Ca{\left( {OH} \right)_2}\]
B: \[CaC{O_3}\]
C: \[CaS{O_4}\]
D: \[HCl\]

Answer 1

Hint: Hard water refers to the water having high content of minerals. Dissolved calcium and magnesium ions in water are the most common types of minerals that lead to hardness of water. The degree of hardness of water increases with increase in the concentration of such cations.

Complete answer:
There are two types of hardness in water namely (i) permanent hardness in water which is due to the presence of chloride as well as sulphate ions for e.g. \[CaC{l_2}\], \[MgC{l_2}\], \[CaS{O_4}\]. It may be removed by the softening process or demineralisation process.
(ii) Temporary hardness which occurs due to the presence of carbonate as well as bicarbonate ions for e.g. \[CaC{O_3}\], \[MgC{O_3}\], \[Ca{\left( {OH} \right)_2}\],\[Mg{(HC{O_3})_2}\]. It may be removed by boiling of the water. Both carbonates and bicarbonates of calcium and magnesium decompose on heating leading to the formation of insoluble carbonate. The precipitation reactions are demonstrated below:
$Ca{(HC{O_3})_2}(aq) \to C{O_2}(g) + {H_2}O(l) + CaC{O_3}(s)$
$Mg{(HC{O_3})_2}(aq) \to C{O_2}(g) + {H_2}O(l) + MgC{O_3}(s)$
The other method used to remove temporary hardness in water is Clark’s method in which bicarbonates of magnesium or calcium react with slaked lime i.e. \[Ca{\left( {OH} \right)_2}\] as demonstrated below:
$Ca{(HC{O_3})_2} + Ca{(OH)_2} \to 2CaC{O_3} + 2{H_2}O$
$Mg{(HC{O_3})_2} + 2Ca{(OH)_2} \to 2CaC{O_3} + Mg{(OH)_2}$
Apart from this, methods to remove temporary hardness in water include washing soda addition, calgon process, using ion exchange resins.

Hence, the correct answer is option A i.e. Temporary hardness may be removed from water by adding \[Ca{\left( {OH} \right)_2}\] (from Clark's method).

Note: Few signs of hard water are listed below:
Chalky or white residue/spots appearing on the dishes.
Strains appear in the shower.
Linens and clothes appear dull and become rough.
Low water pressure from showers owing to the clogged pipes.