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GCSE Chemistry > Water Softening

Temporary Hard Water

This is caused by calcium or magnesium hydrogen-carbonate. It can be removed by boiling the water.

    Ca(HCO3)2 (aqueous)   ---->   CaCO3 (solid)   +   H2O (liquid)   +   CO2 (gas)
This above reaction is the direct opposite of when hard water forms. Boiling water is only an effective method of softening small volumes of water. It is time consuming and expensive for large quantities.

Permanent Hard Water

This is caused by calcium sulphate and CANNOT be reversed by boiling the water. There are two main methods that can be used in this situation.

  • Using Na2CO3 (washing soda) - this can remove both types of hard water...

      Hard Water   +   Washing Soda   ---->   Scale   +   Softened Water

      CaSO4 (aq)   +   Na2CO3 (s)   ---->   CaCO3 (s)   +   Na2SO4 (aq)
      Ca(HCO3)2 (aq)   +   Na2CO3 (s)   ---->   CaCO3 (s)   +   2NaHCO3 (aq)

  • Using an Ion Exchange Column

      Ca2+ (aq)   +   2Na+ (s)   ---->   Ca2+ (s)   +   2Na+ (aq)

    Hard Water passes through a column containing a suitable resin in small granules. The column contains an excess of sodium ions. When the hard water passes through the column, the Ca2+ ions in the water are exchanged for Na+ ions.

    The Na+ ions on the column are recharged at intervals by passing a concentrated solution of sodium chloride through the column. Na+ ions do not cause hardness and so the water is soft. Domestic water softeners work on this principal.

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