Question

Give balanced equations to show

1. how temporary hardness enters into water
2. temporary hardness in water can be removed by boiling.
3. permanent hardness in water can be removed by adding washing soda

Answer 1

(a). Temporary hardness:

Temporary hardness enters water when rainwater containing CO₂ fall on earth and is combined with limestone [CaCO₃] and magnesite [MgCO₃] layer forming respective soluble Bicarbonate.

$\underset{\underset{\left(\mathrm{Insoluble}\right)}{(\mathrm{Calcium}\mathrm{Carbonate})}}{\mathrm{Ca}{\mathrm{CO}}_3\left(\mathrm{s}\right)}+\underset{(\mathrm{Rain}\mathrm{water})}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Carbondioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}\to \underset{\underset{\left(\mathrm{Soluble}\right)}{(\mathrm{Calcium}\mathrm{Bicarbonate})}}{\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{aq}\right)}$

$\underset{\underset{\left(\mathrm{Insoluble}\right)}{(\mathrm{Magnisium}\mathrm{Carbonate})}}{\mathrm{Mg}{\mathrm{CO}}_3\left(\mathrm{s}\right)}+\underset{(\mathrm{rain}\mathrm{water})}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Carbondioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}\to \underset{\underset{\left(\mathrm{Soluble}\right)}{(\mathrm{Magnesium}\mathrm{Bicarbonate})}}{\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{aq}\right)}$

(b). Removal of the temporary hardness of water

• The temporary hardness of water can be removed by boiling.
• The soluble calcium bicarbonate and magnesium bicarbonate decompose on boiling to form their respective insoluble carbonate.
• These insoluble salt can be separated from water by filtration and the water then become soft.

$\underset{\underset{\left(\mathrm{Soluble}\right)}{(\mathrm{Calcium}\mathrm{Bicarbonate})}}{\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{aq}\right)}\stackrel{\mathrm{Boiling}}{\to }\underset{\underset{\left(\mathrm{Insoluble}\right)}{(\mathrm{Calcium}\mathrm{Carbonate})}}{\mathrm{Ca}{\mathrm{CO}}_3\left(\mathrm{s}\right)}+\underset{\left(\mathrm{water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Carbondioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

$\underset{\underset{\left(\mathrm{Soluble}\right)}{(\mathrm{Magnesium}\mathrm{Bicarbonate})}}{\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{aq}\right)}\stackrel{\mathrm{Boiling}}{\to }\underset{\underset{\left(\mathrm{Insoluble}\right)}{(\mathrm{Magnesium}\mathrm{Carbonate})}}{\mathrm{Mg}{\mathrm{CO}}_3\left(\mathrm{s}\right)}+\underset{\left(\mathrm{water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Carbondioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

(c). Removal of the permanent hardness of water

• The permanent hardness of water cannot be removed by boiling as it is caused due to Chloride and sulphate of calcium and magnesium.
• As the salts are stable to heat, permanent hardness can be removed by adding compounds like washing soda (sodium carbonate).
• Washing soda reacts with Chloride and sulphate of calcium and magnesium to form their respective insoluble carbonate, the insoluble carbonate can be filtered out and the water becomes soft.

$\underset{\underset{\left(\mathrm{Soluble}\right)}{(\mathrm{Calcium}\mathrm{Chloride})}}{{\mathrm{CaCl}}_2\left(\mathrm{aq}\right)}+\underset{(\mathrm{Sodium}\mathrm{Carbonate})}{{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)}\to \underset{\underset{\left(\mathrm{Insoluble}\right)}{(\mathrm{Calcium}\mathrm{Carbonate})}}{\mathrm{Ca}{\mathrm{CO}}_3\left(\mathrm{s}\right)}+\underset{\underset{\left(\mathrm{Soluble}\right)}{\left(\mathrm{Carbondioxide}\right)}}{2\mathrm{NaCl}\left(\mathrm{s}\right)}$

$\underset{\underset{\left(\mathrm{Soluble}\right)}{(\mathrm{Magnesium}\mathrm{Sulfate})}}{\mathrm{MgS}{\mathrm{O}}_4(\mathrm{aq}.)}+\underset{(\mathrm{Sodium}\mathrm{Carbonate})}{{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)}\to \underset{\underset{\left(\mathrm{Insoluble}\right)}{(\mathrm{Magnesium}\mathrm{Carbonate})}}{\mathrm{Mg}{\mathrm{CO}}_3\left(\mathrm{s}\right)}+\underset{\underset{\left(\mathrm{Soluble}\right)}{\left(\mathrm{Carbondioxide}\right)}}{{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{s}\right)}$