Describe the hydrolysis of fats to form soaps and the action of soaps.

Saponification is the process of hydrolysis of fat into its constituent glycerol and fatty acids by boiling with alkali (NaOH or KOH)
Soap is the salt of the fatty acid produced.

Saponification of a fat with sodium hydroxide.


3 Na+ ions are required to saponify one fat molecule. These will replace the glycerol, yielding three fatty acids with an Na+ tail.

A soap essentially is a fatty acid salt. Usually it is the sodium salt of a fatty acid; sometimes the potassium or sodium salt. In either case, they are usually made from the hydrolysis of a fat of some kind to form glycerol and the fatty acid.

(non polar, water insoluble end) CH3-(CH2)n - COONa (polar, water soluble end)

external image Rheology_tcm18-34871.gif(read explanation below)

Soaps function because their long non-polar hydrophobic 'tail' dissolves in oil or grease to form a micelle. A micelle is an aggregate with a water-loving (hydrophilic) surface and a fatty core, consisting of a few dozen and up to many thousand molecules, the precise number being given by the aggregation number. Most micelles are spherical, disk-like, or thread-like, but other forms are possible.
This micelle is surrounded by the polar hydrophilic 'heads' of the soap molecules, which make it soluble in water. Soaps don't work well in hard water as the Ca2+ and Mg2+ ions present in the water cause the precipitation of the insoluble Ca or Mg salts of the fatty acids known as 'scum'.