Upper secondary · 20 min

Saponification: turning fat into soap

Boil a fat with sodium hydroxide and watch the triglyceride split into glycerol and fatty-acid salts — soap — which a saturated salt solution then pushes out of the mixture.

Goal

Write the saponification equation, identify soap (RCOONa) and glycerol as products, and understand salting-out as a separation step.

Apparatus and reagents

Vegetable oil or lard, concentrated sodium hydroxide solution, a beaker and stirrer, heat source, saturated sodium chloride solution and a filter.

Procedure

Drag the slider to add sodium hydroxide solution to the warmed fat; the pale specks that appear and sink stand for insoluble soap curds separating out.
Note that glycerol does not precipitate: it stays dissolved in the aqueous layer, which is why the products are easy to separate.
In a real preparation, pour the hot mixture into saturated brine (salting out), skim the soap layer, wash it and let it harden.
Relate the clouding and settling to the equation: fat + 3 NaOH → glycerol + 3 RCOONa; the more base added, the more complete the conversion.

What to observe

As base is added the mixture turns cloudy and pale grains accumulate at the bottom — the fatty-acid salts that make up soap.
Unlike a Ksp precipitate, the soap here separates because it is insoluble in concentrated salt solution, not only because its solubility product is exceeded.

Explanation

A triglyceride is a triester of glycerol with three fatty acids. Hydroxide saponifies each ester bond irreversibly (RCOO⁻ is too weak a base to go back), giving glycerol plus three soap anions. Adding brine lowers the solubility of the sodium soaps, which float or settle as a separate solid phase.

History of the experiment

Saponification is one of the oldest chemical technologies: soaps were already made from fat and plant ashes in ancient Mesopotamia. In 1779 Carl Wilhelm Scheele heated olive oil with litharge and isolated a sweet new substance, glycerol — the other half of the fat molecule. Chevreul’s studies of fats (1810s–1820s) then proved soap is a fatty-acid salt and gave saponification its modern chemistry.

Chemists behind it

Carl Wilhelm Scheele