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GCSE Chemistry > Scum Formation

Soap contains salts such as sodium stearate and sodium palmitate and when calcium ions in hard water mix with soap, a precipitate of calcium stearate / calcium palmitate is formed.

In the following equation, "X" represents either the stearate/palmitate:

    Ca2+ (aq)   +   2X- (aq)   ---->   CaX2 (s)
    Calcium (in hard water)   +   Palmitate/stearate ions in soap   ---->   Scum
Some advantages of hard water as follows:
  • Taste supposedly better
  • Strengthens bones/teeth
  • With correct diet, it can reduce the risk of heart disease
Some disadvantages of hard water:
  • Needs more soap to form a lather
  • Scum formed in wash basins etc...
  • Scale will build up in pipes and reduce the efficiency of machinery using those pipes

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