Soaps and detergents

This page covers one Form 5 content standard from the Consumer and Industrial Chemistry chapter: soaps and detergents. The exam expects you to describe how soap is prepared by saponification, to draw or describe the structure of a soap and a detergent molecule, to compare the two, and to list the additives put into a detergent. The preparation of soap is a classic Paper 2 experiment, so know the reagents, the conditions and the salting-out step precisely.

How soap is made: saponification

Soap is made by saponification, which is the alkaline hydrolysis of an oil or fat. An oil or fat (a triglyceride) is heated with a concentrated alkali, usually sodium hydroxide solution, under reflux. The heat and the alkali break the ester bonds in the triglyceride, giving two products:

• the sodium salt of the fatty acid, which is the soap, and
• glycerol (propane-1,2,3-triol).

In words: fat + sodium hydroxide → soap + glycerol. After the reaction, concentrated sodium chloride (common salt) solution is added to the mixture. Soap is less soluble in salt solution, so it precipitates out as a solid that can be filtered off. This step is called salting out, and it separates the soap from the glycerol and the excess alkali.

If potassium hydroxide is used instead of sodium hydroxide, a softer, more soluble potassium soap is formed, of the kind used in liquid soaps.

The structure of a soap molecule

A soap molecule is the sodium salt of a long-chain carboxylic (fatty) acid, for example sodium stearate, C₁₇H₃₅COO−Na⁺. It has two very different ends:

• a long hydrocarbon tail (the C₁₇H₃₅– chain) that is non-polar and dissolves in oil and grease, it is described as hydrophobic (“water-hating”) or lipophilic; and
• an ionic head (the –COO−Na⁺ end) that is polar and dissolves in water, it is described as hydrophilic (“water-loving”).

This “two-ended” structure is the key to how soap cleans, which is the subject of the next content standard.

What a detergent is

A detergent is a synthetic cleaning agent that does the same job as soap but is made from petroleum (crude oil) fractions, not from natural oils and fats. A long-chain hydrocarbon from petroleum is reacted with concentrated sulfuric acid (sulfonation) and then neutralised with sodium hydroxide, giving the sodium salt of a sulfonic acid, for example a sodium alkylbenzenesulfonate. Like soap, a detergent molecule has a hydrophobic hydrocarbon tail and a hydrophilic ionic head (here the –SO₃−Na⁺ group).

Comparing soaps and detergents

The most examined difference is their behaviour in hard water, which contains dissolved calcium (Ca₂⁺) and magnesium (Mg₂⁺) ions:

• Soap reacts with these ions to form an insoluble precipitate called scum, wasting soap before any cleaning happens.
• Detergent does not form scum, because its calcium and magnesium salts are soluble. This is why detergents lather and clean well even in hard water.

Other points of comparison: soap is made from a renewable resource (plant and animal oils) and is biodegradable; many early detergents were made from non-renewable petroleum and some were non-biodegradable, though modern detergents are designed to break down.

Additives in detergents

A commercial detergent powder is a mixture. Besides the active detergent, it contains additives, each with a purpose you should be able to state:

• Biological enzymes, break down protein, starch and fat stains at low temperature.
• Whitening agents (optical brighteners), reflect light to make fabrics look brighter and whiter.
• Water softeners / builders (for example sodium tripolyphosphate), remove Ca²⁺ and Mg²⁺ ions so the detergent works better.
• Fragrances (perfumes), give a pleasant smell.
• Bleaching agents, remove coloured stains by oxidation.
• Suspension (anti-redeposition) agents, keep loosened dirt suspended so it does not settle back on the cloth.

Worked example

Question. During saponification, one mole of a fat (a triglyceride) reacts completely with sodium hydroxide according to the ratio 1 fat : 3 NaOH → 3 soap + 1 glycerol. A student uses 0.20 mol of the fat. Calculate (a) the number of moles of sodium hydroxide needed, and (b) the number of moles of soap and of glycerol produced.

Step 1, Write the mole ratio. From the equation, fat : NaOH : soap : glycerol = 1 : 3 : 3 : 1.

Step 2, Find the moles of NaOH. Moles of NaOH = 3 × moles of fat = 3 × 0.20 mol = 0.60 mol.

Step 3, Find the moles of products. Moles of soap = 3 × 0.20 mol = 0.60 mol. Moles of glycerol = 1 × 0.20 mol = 0.20 mol.

Answer. (a) 0.60 mol of sodium hydroxide is needed. (b) 0.60 mol of soap and 0.20 mol of glycerol are produced.

Practice question

Question. (a) Name the process used to make soap from palm oil, and state the two reagents needed. (b) State why concentrated sodium chloride solution is added at the end. (c) Give one reason a detergent may be preferred to soap for washing clothes in hard water.

Answer. (a) The process is saponification; the reagents are the oil or fat (palm oil) and a concentrated alkali (sodium hydroxide solution). (b) Sodium chloride is added to precipitate the soap (salting out), because soap is less soluble in the salt solution and separates out as a solid. (c) A detergent does not form scum with the calcium and magnesium ions in hard water, its calcium and magnesium salts are soluble, so it lathers and cleans better and no cleaning agent is wasted.

Exam tip

For the preparation, always give three things: the oil or fat, the concentrated alkali (NaOH), and the condition heat (under reflux), then the salting-out step with sodium chloride. Do not confuse the products: soap + glycerol, not “soap + water”. When comparing soap with detergent, the scoring difference is scum in hard water: soap forms it, detergent does not. If a molecule diagram is asked for, label the hydrophobic tail and the hydrophilic head clearly.

Where this fits

This is content standard 13.2 of the Consumer and Industrial Chemistry chapter. It follows directly from oils and fats, the raw materials of soap, and leads into the cleaning action you study next. Reinforce it with the chapter revision notes and practise the saponification and comparison questions in the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, with a paid one-hour trial, a one-to-one teacher can check that your preparation answer includes every reagent, condition and the salting-out step, which is where marks in 4541 are most often dropped.

Quick recap

• Soap = sodium salt of a fatty acid, made by saponification (oil/fat + concentrated NaOH, heated), then salting out with NaCl; glycerol is the by-product.
• Detergent = synthetic cleaning agent from petroleum, a sodium sulfonate salt.
• Both molecules have a hydrophobic tail and a hydrophilic head.
• Soap forms scum in hard water; detergent does not.
• Detergent additives: enzymes, optical brighteners, water softeners, fragrances, bleaches, anti-redeposition agents.