Soaps – SS3 Chemistry Lesson Note (First Term)

Soap is a salt of a fatty acid, made by saponification, in which fats or oils react with a strong alkali such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) to give soap and glycerol. Each soap molecule has a hydrophilic head and a hydrophobic tail, so it can interact with both water and grease.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define soap and saponification.
2. describe the preparation of soap, with its equation.
3. draw and label the structure of a soap molecule.
4. explain the cleansing action of soap using micelles.
5. explain why soap forms scum in hard water.

Instructional techniques

• Identification
• Explanation
• Questions and answers
• Demonstration
• Videos from the source

Instructional materials

• Videos
• Loudspeaker
• Textbook
• Pictures
• Vegetable oil
• Caustic soda or potash
• Wood ash
• Containers/reaction vessels

What is saponification?

Saponification is the alkaline hydrolysis of a fat or oil to give soap and glycerol.

• Fats and oils: esters of glycerol (propane-1,2,3-triol) and long-chain fatty acids, called triglycerides.
• Alkali: a strong base such as NaOH (caustic soda) or KOH (potash).
• Products: soap (the sodium or potassium salt of the fatty acid) and glycerol.

How is soap prepared?

Soap is prepared by heating a fat or oil with a strong alkali, which is the saponification reaction.

Word equation

Triglyceride + 3NaOH → Glycerol + 3 soap molecules

Example with coconut oil

Coconut oil (a triglyceride) + 3NaOH → Glycerol (C₃H₈O₃) + 3 sodium fatty acid salts (soap)

• Coconut oil: a triglyceride containing a mixture of fatty acids.
• Sodium hydroxide: three molecules react with each triglyceride molecule.
• Products: one glycerol molecule and three molecules of sodium fatty acid salts, which are the soap.

Steps in making soap

1. Measure the oil (vegetable oil or palm oil) into a container.
2. Dissolve caustic soda in water. Add the caustic soda to the water, never water to caustic soda. Alternatively, soak wood ash in water and filter to get potash solution.
3. Warm the oil and add the alkali solution slowly, stirring all the time.
4. Heat and stir until the mixture thickens. Saponification is taking place.
5. Add concentrated sodium chloride (common salt) solution. The soap separates and floats. This is called salting out.
6. Skim off the soap, wash it, press it into moulds and leave it to dry.

Safety: caustic soda burns the skin and eyes. Wear gloves and eye protection.

What are the types of soap?

There are two types of soap, hard soap and soft soap, depending on the alkali used.

What is the structure of a soap molecule?

A soap molecule has two main parts: a hydrophilic (water-attracting) “head” and a hydrophobic (water-repelling) “tail”.

• Head: the ionic carboxylate group (for example –COO^–Na⁺), which dissolves in water.
• Tail: the long hydrocarbon chain, which is repelled by water but attracted to oil and grease.
• Dual nature: this lets soap interact with both water and oils, which makes it effective in cleaning.

Because of this structure, soap can emulsify and lift away oily or greasy substances in the presence of water.

How does soap act as an emulsifying agent?

An emulsifying agent is a substance that helps oil and water mix and stay mixed. Soap does this because of its molecular structure.

Steps in the cleansing action of soap

1. Hydrophilic head interaction: the head is attracted to water molecules, so it dissolves in water.
2. Hydrophobic tail interaction: the long tail is repelled by water but attracted to oils and grease.
3. Formation of micelles: in water containing grease, soap molecules arrange themselves into micelles.
4. Encapsulation of oil: the tails embed into the oil or grease and form a core inside the micelle.
5. Dispersion in water: the heads face outward and interact with water, so the whole micelle stays suspended.
6. Rinsing: the stable emulsion is washed away, carrying the trapped grease with it.

• Micelle: a cluster of soap molecules with tails pointing inward and heads pointing outward.
• Tails in a micelle: they cluster together in the centre, away from water.
• Heads in a micelle: they face outward, towards water molecules.

Why does soap form scum in hard water?

Soap forms scum in hard water because calcium and magnesium ions react with soap to form an insoluble solid.

• Hard water: water containing dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) salts.
• Scum: the insoluble calcium or magnesium salt of the fatty acid, a greyish curd.
• Effect: soap is wasted and little lather forms until all the hardness ions are removed.

2C₁₇H₃₅COO^–Na⁺ + Ca²⁺ → (C₁₇H₃₅COO)₂Ca + 2Na⁺

What is the difference between soap and a soapless detergent?

Soap is the sodium or potassium salt of a fatty acid, made from fats or oils and alkali.

A soapless detergent is a synthetic cleaning agent, usually made from petroleum-based chemicals, that also has a hydrophilic head and hydrophobic tail.

What are the uses of soap?

Soap is used mainly for cleaning, and it has other uses too.

• Washing: cleaning the body, clothes and dishes.
• Emulsifying: mixing oil and water in some industrial processes.
• By-product use: the glycerol from saponification is used in cosmetics and medicines.

Evaluation

1. Define saponification and write the word equation for the preparation of soap.
2. State the steps in making soap in the laboratory or at home.
3. Describe the functions of soap as an emulsifying agent.
4. Why does soap form scum in hard water?
5. State two differences between hard soap and soft soap.

Assignment

1. Draw a soap molecule and label its hydrophilic head and hydrophobic tail.
2. Explain how micelles form when soap is added to water containing grease.