Question

Explain with the equation, what is noticed when permanent hard water is treated
with (a) slaked lime and (b) washing soda.

Answer 1

(a) Equation when permanent hard water is treated with slaked lime:

• Hard water contains Chloride and sulfates of Magnesium($\mathrm{Mg}$) and Calcium ($\mathrm{Ca}$).
• Magnesium Chloride (${\mathrm{MgCl}}_2$) upon treatment with slaked lime($\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$) results in the formation of Magnesium hydroxide($\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$) and Calcium chloride(${\mathrm{CaCl}}_2$)
• $\underset{\mathrm{Magnesium}\mathrm{chloride}}{{\mathrm{MgCl}}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Slaked}\mathrm{lime}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)}\to \underset{\mathrm{Magnesium}\mathrm{hydroxide}}{\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)}+\underset{\mathrm{Calcium}\mathrm{chlorides}}{\mathrm{Ca}{\left(\mathrm{Cl}\right)}_2\left(\mathrm{aq}\right)}$
• Magnesium hydroxide($\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$) will settle down.
• The calcium ion of hard water can not be replaced by the calcium ion of slaked lime, thus chloride or sulfate of calcium of hard water can not be removed using slaked lime.

(b) Equation when permanent hard water is treated with washing soda:

• Magnesium Chloride (${\mathrm{MgCl}}_2$) upon treatment with washing soda(${\mathrm{Na}}_2{\mathrm{CO}}_3$) results in the formation of precipitates of Magnesium carbonate (${\mathrm{MgCO}}_3$) and Sodium chloride($\mathrm{NaCl}$).
• $\underset{\mathrm{Magnesium}\mathrm{chloride}}{{\mathrm{MgCl}}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Washing}\mathrm{soda}}{{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)}\to \underset{\mathrm{Magnesium}\mathrm{carbonate}}{{\mathrm{MgCO}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{chloride}}{2\mathrm{NaCl}\left(\mathrm{aq}\right).}$
• Calcium sulfate(${\mathrm{CaSO}}_4$) upon treatment with washing soda(${\mathrm{Na}}_2{\mathrm{CO}}_3$) results in the formation of precipitates of Calcium carbonate (${\mathrm{CaCO}}_3$) and Sodium sulfate(${\mathrm{Na}}_2{\mathrm{SO}}_4$).
• $\underset{\mathrm{Calcium}\mathrm{sulfate}}{{\mathrm{CaSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Sodium}\mathrm{carbonate}}{{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)}\to \underset{\mathrm{Calcium}\mathrm{carbonate}}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{sulfate}}{{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right).}$