To understand how soap forms it is useful to first look at the structure of fats. In chemistry fats belong to a group of compounds called esters. Esters are formed when a carboxylic acid reacts with an alcohol. During this reaction part of the acid combines with part of the alcohol and a molecule of water is released. Because water is produced this reaction is called a condensation reaction.

Fats form in exactly this way. The alcohol involved is called glycerin or glycerol. This molecule is special because it contains three hydroxyl groups. These groups allow glycerol to react with three carboxylic acids at the same time.

These acids are called fatty acids. When glycerol reacts with three fatty acids three ester bonds are created and a large molecule called a triglyceride forms. Triglycerides are the main chemical form of fats and oils in food.

Saponification happens when these fat molecules react with a strong base such as sodium hydroxide. The base breaks the ester bonds and splits the triglyceride apart.

Two main products form during this reaction. One is glycerol which is the alcohol that originally formed the backbone of the fat molecule. The other products are salts of the fatty acids. These fatty acid salts are what we call soap.

Soap molecules are effective cleaning agents because one end interacts well with fats while the other end interacts well with water. When soap meets grease many molecules surround small droplets of fat. The fat becomes dispersed in the water and can then be washed away.