Get premium membership and access revision papers with marking schemes, video lessons and live classes.

Processing. Please wait.

Types of water hardness:temporary hardness of water and how it is removed

Answer Text:
Types of water hardness:Temporary hardness
- Is hardness due to the presence of #CaHCO_3# or# Mg(HCO_3)_2# in water; and can usually be removed by boiling.
Removal of temporary hardness in water:
Boiling:
- Boiling decomposes and an insoluble chalk of #CaCO_3# and #MgCO_3#respectively is deposited in the sides of the vessel.
- This forms an encrustation commonly known as furr the process being furring.
Equations: #Ca(HCO_3)_2(s) to CaCO_3(s) + 2CO_2(g) +H_2O(l)#
#Mg(HCO_3)_2(s)to MgCO_3(s) + 2CO_2(g) +H_2O(l)#
Distillation:
- Water containing dissolved salts is heated in a distillation apparatus;
- Pure water distils over first leaving dissolved salts in the distillation flask (refer to separation of mixtures)
- Is of less economic value as it is too expensive hence disadvantageous.
Addition of calcium hydroxide:
- Involves adding correct amount of lime water where #CaCO_3# is precipitated out.
- This method is cheap and can be used on large scale at water treatment plants.
- However if excess lime (#Ca^(2+)#) ions is added this will make water hard again.
Equation:
#Ca(HCO_3)_2(aq) +Ca(OH)_2(aq)to 2CaCO_3(s) +2H_2O(l).#
Addition of ammonia
solution:
- Addition of aqueous ammonia to water containing calcium and magnesium hydrogen carbonates (temporary hard) precipitates calcium and magnesium ions as corresponding carbonates.
Equations:
#Ca(HCO_3)_2(aq) +2NH_4OH(aq) to CaCO_3(s)+2H_2O(l) + (NH_4)_2CO_3(aq)#
By permutit softener (ion exchange).
- Uses a complex sodium salt (NaX), such as sodium aluminium silicate commonly known as sodium permutit.
- Permutit is a manufactured ion exchange resin.
Iron exchange resin: materials that will take ions of one element out of it’s compounds and replace it with ions another element
Working principle
- The permutit is contained in a metal cylinder
- The hard water is passed through the column of permutit in the cylinder and it emerges softened at the other end
- As hard water passes through the column ion exchange takes place.
- The #Ca^(2+) and Mg^(2+)# remain in the column while sodium ions from the permutit pass into water thus softening it.
Diagram: permutit water softener.
Equations:
#NaX(aq) + Ca^(2+)(aq)to CaX(s) + 2Na^(+)(aq) NaX(aq) + Mg^(2+)(aq) MgX(s) + 2Na^(+)(aq)#
- When all the Na+ ions in the permutit have been replaced by #Ca^(2+) and Mg^(2+)# ions the permutit cannot go on softening water.
- It is then regenerated by washing the column with brine (a strong NaCl solution); during which calcium and magnesium chlorides are washed away.
Equation:
#CaX(s) + 2NaCl(aq) CaCl_2(aq) + Na_2X(s) MgX(s) + 2NaCl(aq) MgCl_2(aq) + Na_2X(s)#