Carboxylic acids and related compounds

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Why doesn’t hard water produce a lather with soap?

Hard water contains ions such as those of magnesium, Mg²⁺ and calcium, Ca²⁺.
 
Soap contains the sodium (or potassium) salts of complex carboxylic acids, e.g. sodium stearate.
 
Although the sodium (or potassium) salts of these acids are soluble in water, the corresponding salts of magnesium and calcium are insoluble. When soap comes into contact with hard water, a precipitation reaction occurs, and these insoluble salts are produced as a 'scum'. The soap is therefore prevented from forming a lather and from carrying out its washing functions until all the magnesium and calcium ions have been removed by this precipitation process.
 
Igloo

Risk assessment
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updated: 03 December 2003