Experiment: Preparation of soap by saponification

Making soap from a vegetable oil is the classic Form 5 practical for the Consumer and Industrial Chemistry chapter, and it shows chemistry that has been used for centuries still working in the laboratory. In this experiment we prepare soap by heating oil with an alkali, a reaction called saponification, and then separate the solid soap by salting out. This guide gives the method, the observations to expect, the reasoning behind each step, and the science process skills the practical papers reward.

Aim

To prepare soap by saponification, that is, by boiling a vegetable oil (or animal fat) with concentrated sodium hydroxide solution, and to separate the soap produced.

Apparatus and materials

• Palm oil or other vegetable oil (or an animal fat)
• Concentrated sodium hydroxide solution
• Saturated sodium chloride (common salt) solution
• Beaker (250 cm³) and glass stirring rod
• Bunsen burner, tripod, wire gauze and heatproof mat
• Measuring cylinder
• Filter funnel and filter paper
• Ethanol (to help the oil and alkali mix)
• Distilled water

Procedure

1. Measure a fixed volume of vegetable oil into a beaker.
2. Add concentrated sodium hydroxide solution to the oil, and a little ethanol to help the oil and the alkali mix together.
3. Heat the mixture gently and stir continuously with the glass rod. Keep the mixture from boiling over and add a little distilled water if it becomes too thick.
4. Continue heating and stirring for several minutes until the oily layer disappears and a thick, uniform paste forms. This shows saponification is taking place.
5. Stop heating and allow the mixture to cool slightly.
6. Add saturated sodium chloride solution to the warm mixture and stir. A solid layer of soap separates and rises to the surface.
7. Filter the mixture to collect the solid soap, or scoop off the floating soap.
8. Wash the soap with a little cold distilled water to remove any excess alkali, and press it dry between sheets of filter paper.

Expected observations

At first the oil and the alkali form separate layers that do not mix easily. On heating and stirring with a little ethanol, the two combine and the oily layer gradually disappears, leaving a thick, creamy, uniform paste, the sign that the oil has reacted. When saturated salt solution is added, a solid layer of soap forms and floats to the top of the liquid, while the liquid below contains glycerol, the salt and any excess alkali. The collected soap is a soft solid that lathers when rubbed with water. Describe these qualitative changes, the oily layer disappearing, the paste forming, the soap floating out on adding salt, rather than quoting exact masses of soap you have not weighed.

Inference and conclusion

Saponification is the reaction between a fat or oil and a hot concentrated alkali to produce soap and glycerol. The oil is an ester of long-chain fatty acids and glycerol; the sodium hydroxide breaks these ester links (hydrolysis) and forms the sodium salt of the fatty acid, which is the soap, together with glycerol. The soap is soluble in water, so it does not separate on its own; adding saturated sodium chloride solution lowers its solubility and makes it separate as a solid that floats, this is salting out. The liquid left behind holds the glycerol and excess alkali, which is why the soap is washed to remove leftover alkali. The conclusion is that soap can be prepared by saponifying a vegetable oil with concentrated alkali, and that salting out is the method used to collect the solid soap.

Science process skills (Paper 3 style)

Operational definition of saponification complete. Saponification is defined operationally as complete when the separate oily layer has disappeared and the mixture has become a single thick, uniform paste.

Identifying variables. If the investigation compared how much soap different oils give, the manipulated variable is the type of oil, the responding variable is the mass or amount of soap collected, and the controlled variables are the volume of oil, the concentration and volume of alkali, and the heating time.

Classifying steps by purpose. Classify each step: reaction (heating oil with alkali), separation (salting out and filtering), and purification (washing the soap). This shows you understand why each step is done.

Making an inference. From the disappearance of the oily layer, infer that the ester has reacted with the alkali to form soap and glycerol.

Communicating. Write a conclusion that names the reaction (saponification), the products (soap and glycerol), and the reason salt is added (to salt out the soap).

Safety precautions

• Wear safety goggles, because concentrated sodium hydroxide is very corrosive and can cause serious eye damage.
• Handle the concentrated alkali with care and wipe up any spills at once, so it does not burn the skin.
• Heat gently and stir to stop the mixture spitting or boiling over, so hot alkaline paste does not splash out.
• Keep ethanol away from the open flame while adding it, because ethanol is flammable; add it before heating, not to a hot mixture near the burner.
• Let hot apparatus cool before handling, or use tongs, to avoid burns.

Common errors

• Not heating long enough. If the mixture is heated too briefly, the oily layer remains and saponification is incomplete; keep heating and stirring until a uniform paste forms.
• Forgetting the ethanol. Without a little ethanol the oil and alkali do not mix well and the reaction is slow; add ethanol to help them combine.
• Using dilute alkali. Dilute sodium hydroxide reacts too slowly; concentrated alkali is needed for saponification.
• Skipping the salting out. The soap is soluble and will not separate on its own; adding saturated salt solution is essential to make the solid soap float out.
• Not washing the soap. Soap that still holds excess alkali is caustic; wash it with a little cold water before use.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can explain, not just perform, every step, why concentrated alkali, why ethanol, why salt is added at the end, because Paper 2 and Paper 3 both reward the reason behind the method. Saponification and the structure of soap run through the Consumer and Industrial Chemistry chapter of SPM Chemistry, and this practical prepares the follow-up experiment on the cleaning action of soaps and detergents. It also trains exactly the operational thinking assessed in the Paper 3 practical (Paper 3 is a practical test assessing science process skills): defining when a reaction is complete by what you can see, and justifying each separation and purification step.