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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