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GCSE Chemistry > Hard Water

Hard water does not produce as much lather as soft water when it is used with soap and also an insoluble solid called scum is left on the sides of the wash basin.

Calcium chloride, Magnesium nitrate, and Iron (II) Sulphate produce scum and no lather when contained in a sample of water that is mixed with soap.

The ions that are responsible for "Hard Water" are therefore as follows:

  • Ca2+
  • Mg2+
  • Fe2+
Hard water is formed by 2 main methods:
  • H2O (l) [In rain]   +   CO2 (g) [In air]   ---->   H2CO3 (aq) [Carbonic Acid]

    This carbonic acid flows over chalk / limestone - reacts with calcium to form Calcium Hydrogen-carbonate. This is now soluble in water and so can cause hard water

  • CaCO3 (s)   +   H2CO3 (aq)   ---->   Ca(HCO3)2 (aq) [Calcium Hydrogen-carbonate]

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