Question

Calcium bicarbonate causes hardness in water but not calcium carbonate. Why?

Answer 1

To answer we should know the solubility rules. The main salts present in hard water are sulphate, halides and bicarbonates of alkaline earth metals. The salt which is soluble in water will produce ions. The salt which is insoluble in water will not produce any ions. We will check the solubility of salt in water.

Complete answer:
On the basis of the concentration of ions, particularly calcium and magnesium, the water is divided into two parts: Hard water and soft water.
Hard water: the water contains a high amount of dissolved salts. Hard water has a multivalent cation. The commonly present cations are calcium ${\text{C}}{{\text{a}}^{{\text{2 + }}}}$and magnesium${\text{M}}{{\text{g}}^{{\text{2 + }}}}$. The ions enter into the water for leaching. The ions are mixed into the water in the form of their minerals. The bicarbonate produces temporary hardness. Sulphate and halides produced permanent hardness.
According to the solubility rules, the bicarbonates are soluble so, calcium bicarbonate is soluble in water. It dissociates in water and produces calcium ions which is the cause of hardness.
The dissociation of calcium bicarbonate in water is as follows:
${\text{CaHC}}{{\text{O}}_{\text{3}}}\, \to \,{\text{C}}{{\text{a}}^{{\text{2 + }}}}{\text{(aq)}}\,{\text{ + }}\,{\text{HCO}}_3^ - {\text{(aq)}}$
According to the solubility rules, the carbonates are insoluble so, calcium carbonate is insoluble in water. It does not dissociate in water and does not produce the calcium ions which is the cause of hardness.

Therefore, calcium bicarbonate causes hardness because it is soluble in water but calcium carbonate does not because it is insoluble in water.

Note: Soft water contains monovalent cations such as sodium chloride and potassium chloride whereas hard water contains multivalent cations. The hard water can be converted into soft water by the ion exchange method, permutit method or by using calgon’s method. In all methods, generally ions producing hardness get exchanged with sodium ions. Permuted method is used for filtration of hard water. Filtration removes the impurities. Permutit is an artificial zeolite. Permutit has a porous structure. The pores can trap ions of specific size.