Saponification boils a fat or oil (an ester) with concentrated sodium hydroxide to make soap and glycerol, for example (C17H35COO)3C3H5 + 3NaOH → 3C17H35COONa + C3H5(OH)3. Salt is then added to precipitate the soap.
Soap is made by saponification, the alkaline hydrolysis of a fat or oil. Fats and oils are esters of glycerol (propane-1,2,3-triol) with long-chain fatty acids, and boiling them with sodium hydroxide breaks the ester and produces the sodium salt of the fatty acid, which is soap, together with glycerol. This reaction is the heart of the soap section in the Form 5 Consumer and Industrial Chemistry chapter, and the exam tests the equation, the conditions and the salting-out step.
The balanced equation
Taking glyceryl tristearate, a typical fat, reacting with sodium hydroxide:
(C17H35COO)3C3H5(l) + 3NaOH(aq) → 3C17H35COONa(aq) + C3H5(OH)3(aq)
Here (C17H35COO)3C3H5 is the fat (an ester of glycerol), C17H35COONa is sodium stearate, the soap, and C3H5(OH)3 is glycerol. The mole ratio is the key: one fat molecule reacts with three sodium hydroxide because the fat has three ester linkages, giving three soap molecules and one glycerol. In general terms:
fat or oil + 3NaOH → 3 soap + glycerol
A word equation is accepted where the full formula is not required, but the 1 : 3 ratio of fat to alkali must be right.
Conditions required
- Concentrated sodium hydroxide solution is the reagent.
- The mixture is heated (boiled), often under reflux, to hydrolyse the ester.
- Saturated sodium chloride solution (brine) is added at the end to precipitate the soap.
Observations
- On boiling the fat with alkali, the mixture becomes a thick, cloudy paste as soap forms.
- When saturated salt solution is added, solid soap separates and floats to the top, this is salting out, and can be filtered off.
- The lower layer contains glycerol and any excess alkali in solution.
Soap is much less soluble in concentrated salt solution than in water, which is why adding brine forces it out as a solid.
Saponification as the reverse of esterification
Saponification is an alkaline hydrolysis of an ester, so it is essentially the reverse of esterification: esterification joins an acid and an alcohol into an ester and water, while saponification splits an ester (the fat) with alkali back into a carboxylate salt and glycerol. Seeing the two reactions as a pair helps you remember both, and it is a link the exam sometimes rewards.
How soap works, in one line
The soap molecule has a long non-polar hydrocarbon tail that dissolves in grease and an ionic -COO− Na+ head that dissolves in water. This dual nature lets soap lift grease off cloth and hold it in water as an emulsion, which is why the same chapter follows saponification with cleaning action. Knowing the structure of the soap ion links the making of soap to how it cleans.
Common mistakes to avoid
Use the 1 : 3 ratio of fat to sodium hydroxide, a very common error is to write 1 : 1. Remember there are two products, soap and glycerol; do not omit the glycerol. Name the salt as the sodium salt of the fatty acid (soap), and state that concentrated sodium hydroxide is boiled with the fat. Do not confuse the salting-out sodium chloride with the sodium hydroxide reagent.
How it appears in the SPM exam
In Paper 2 (4541/2) you may write the saponification equation, name the two products, state the conditions, and explain the salting-out step. In Paper 1 (4541/1), objective items test the reagent, the products and the purpose of adding salt. In Paper 3 (4541/3), preparing soap by boiling oil with sodium hydroxide and salting it out is a classic practical. Anchor your answer on fat + alkali → soap + glycerol, and the whole soap-making question follows.
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