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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- When all the Na+ ions in the permutit have been replaced by `Ca^(2+) and Mg^(2+)` ions the permutit cannot go on softening water.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)`