while removing the hardness of water, Magnesium turns into Magnesium Hydroxide and not MAgnesium Carbonate while Calcium turns into Calcium carbonate. Why does magnesium form hydroxide instead of carbonate?

Asked by devyani.gauri99 | 29th Dec, 2015, 11:22: PM

Expert Answer:

In the process called softening of water, slaked lime (Ca(OH)2) is added to the hard water.

As calcium hydroxide is insoluble in water, calcium has to be precipitated as CaCO3.

To remove carbon dioxide in water:

H2CO3 + Ca(OH)2 → CaCO3 + 2H2O

To remove calcium temporary hardness:

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O

To remove magnesium temporary hardness, add lime and more lime,

Stage 1

Mg(HCO3)2 + Ca(OH)2 → MgCO3 + CaCO3 + 2H2O

The calcium carbonate precipitates, but the magnesium carbonate does not as per its Ksp, hence further lime is added.

Stage 2

MgCO3 + Ca(OH)2 → Mg(OH)2 + CaCO3

Combining above two equations:

Mg(HCO3)2 + 2Ca(OH)2 → 2CaCO3↓ + Mg(OH)2↓ + 2H2O

The magnesium hydroxide and calcium carbonate precipitate.

Answered by Prachi Sawant | 2nd Jan, 2016, 09:45: AM