Question

Reaction of Calgon with hard water containing $C{{a}^{2+}}$ ions produces:
(A) ${{[N{{a}_{2}}Ca{{P}_{6}}{{O}_{18}}]}^{2-}}$
(B) $C{{a}_{2}}{{(P{{O}_{4}})}_{3}}$
(C) $CaC{{O}_{3}}$
(D) $CaS{{O}_{4}}$

Answer 1

Hint: Hard water consists of magnesium and calcium ions. Water softeners are used to remove these ions. Calgon is one such substance which reacts with calcium and magnesium ions and forms a complex such that these ions are removed from water hence removing the hardness of the water. Find the structure of compound Calgon in order to write the chemical reaction.

Complete step by step solution:
Hard water is the water having high mineral content in contrast to soft water. Hard water is formed when water percolates through deposits of ores like limestone, chalk or gypsum which contain the ions of calcium and magnesium.
Hardness of water can be of two types namely, Temporary and permanent hardness.
Temporary hardness is a type of water hardness caused by the presence of dissolved salts of bicarbonate minerals like calcium bicarbonate and magnesium bicarbonate. Temporary hardness can be removed by simple heating of the solution or through the process of lime softening.
Permanent hardness contains mainly salts like calcium sulphate and magnesium sulphate. These salts cannot be removed by simple boiling. This is because these salts do not precipitate when the solution is heated.
This is the reason why we use Calgon. Calgon is a chemical compound with the composition sodium hexametaphosphate.
The chemical reaction taking place is given below:
${{[N{{a}_{4}}{{P}_{6}}{{O}_{18}}]}^{2-}}\text{ + }C{{a}^{2+}}\text{ }\to \text{ }\!\![\!\!\text{ }N{{a}_{2}}Ca{{P}_{6}}{{O}_{18}}{{]}^{2-}}$

Therefore, the correct answer is option (A).

Note: Similar to the way Calgon reacts with calcium ions in hard water, it reacts with magnesium ions and forms a complex removing the hardness of the water. The reaction is given below:
${{[N{{a}_{4}}{{P}_{6}}{{O}_{18}}]}^{2-}}\text{ + M}{{\text{g}}^{2+}}\text{ }\to \text{ }\!\![\!\!\text{ }N{{a}_{2}}Mg{{P}_{6}}{{O}_{18}}{{]}^{2-}}$