Soaps in Hard Water

Students learn to distinguish between soaps and synthetic detergents in terms of:
  • the structure of the molecule
  • chemical composition
  • effect in hard water

Soaps and other surfactants have very different properties in hard water. The reason for this is that soaps have a carboxylate end-group, which is a result of the hydrolysis of fatty acid. The carboxylate end-groups turn out to be particularly good at complexing (i.e. bonding to) metal ions, especially those found in hard water (e.g. calcium ions).

This effect is all too familiar for our friends in Adelaide - the complexed metal ions (usually calcium) tend to precipitate out, giving the water a cloudy, murky appearance. (The cloudiness is due to the precipitation of surfactants, while the murky, brown colour is due to tannins in the water, just like weak tea.)

Even more exciting for the residents of Adelaide, this precipitate can form a scum on the surface of the bath, the basin and anybody who steps into the water.

One of the driving forces for the development of detergents was the need for molecules with soap-like cleaning properties that were insensitive to metal ions in the water.