Soft water does not contain ions that produce scum or scale.
Hard water can be softened by removing the ions that produce scum and scale.
Temporary hardness is removed from water by heating it. Permanent hardness is not changed by heating.
2HCO3- (aq) --> CO32- (aq) + H2O (l) + CO2 (g)
The hydrocarbonate ions in temporary hard water decompose on heating. The carbonate ions formed react with Ca2+ (aq) and Mg2+ (aq) ion, making precipitates.
Either type of hard water can be softened by adding washing soda or by using an ion-exchange resin to remove calcium and magnesium ions.
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