What is hard water? How can you remove hardness of water by Calgon method.

Hard water : Water which does not give good lather readily with soap is called hard water.
Calgon method : Sodium hexametaphosphate is commercially called Calgon. When Calgon is added to the hard water it react with calcium and magnesium ions forming complex anions.
${\mathrm{Na}}_6{\mathrm{P}}_6{\mathrm{O}}_{18}\stackrel{ }{\to }2{\mathrm{Na}}^{+}+{\mathrm{Na}}_4{\mathrm{P}}_6{\mathrm{O}}_{18}^{2-}$
${\mathrm{M}}^{2+}+{\mathrm{Na}}_4{\mathrm{P}}_6{\mathrm{O}}_{18}^{2-} \stackrel{ }{\to } {\left[{\mathrm{Na}}_2{\mathrm{MP}}_6{\mathrm{O}}_{18}\right]}^{2-}+2{\mathrm{Na}}^{+}[\mathrm{M}=\mathrm{Mg},\mathrm{Ca}]$
The complex anion keeps the Mg²⁺ and Ca²⁺ ions in solution.