How do soaps and detergents clean, and how do emulsifiers keep mixtures stable?
The production of soap by alkaline hydrolysis of fats and oils, the structure of soap with hydrophilic and hydrophobic ends, how soap removes oily dirt, hard water, detergents, and the action of emulsifiers in emulsions.
An SQA Higher Chemistry answer on soaps, detergents and emulsions, covering how soap is made by alkaline hydrolysis of fats and oils, the hydrophilic and hydrophobic ends of a soap ion, how oily dirt is removed, the advantage of detergents in hard water, and how emulsifiers stabilise emulsions.
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What this key area is asking
The SQA wants you to describe how soap is made from fats and oils, explain how a soap ion removes oily dirt using its hydrophilic and hydrophobic ends, explain the advantage of detergents in hard water, and describe how an emulsifier stabilises an emulsion. The hydrophilic-and-hydrophobic cleaning mechanism is a near-certain exam item and must be explained in full.
Making soap
How soap cleans
A soap ion has two parts with opposite behaviour:
The oily dirt is held in a ball-like structure (a micelle) with the hydrophobic tails pointing in and the hydrophilic heads pointing out into the water, keeping the dirt suspended so it can be rinsed away.
Hard water and detergents
In hard water soaps react with dissolved calcium and magnesium ions to form an insoluble scum. Detergents are synthetic cleaning agents that work like soap, with a hydrophilic head and a hydrophobic tail, but do not form scum in hard water, so they clean effectively where soap fails.
Worked example: soap from hydrolysis
Emulsions and emulsifiers
An emulsion is a mixture of small droplets of one liquid suspended in another, such as oil in water. An emulsifier stabilises an emulsion because each molecule has a hydrophilic end that sits in the water and a hydrophobic end that sits in the oil, stopping the two layers separating. Emulsifiers are widely used in foods to keep oil and water mixed.
Examples in context
The cleaning chemistry here is the same whether you are washing dishes or cleaning up an oil spill: surfactant molecules with a water-loving head and an oil-loving tail surround the grease and lift it into the water. Synthetic detergents replaced soap in most laundry products precisely because they do not form scum in the hard water common across much of Britain. Emulsifiers are everywhere in the food cupboard: lecithin from egg yolk keeps the oil and vinegar in mayonnaise from separating, and emulsifiers in ice cream give it a smooth texture by keeping fat droplets evenly dispersed. The human body uses the same principle, with bile salts acting as natural emulsifiers that break dietary fat into tiny droplets so that lipase enzymes can hydrolyse it efficiently.
Try this
Q1. Name the two products of the alkaline hydrolysis of a fat to make soap. [2 marks]
- Cue. Glycerol and the sodium (or potassium) salts of the fatty acids.
Q2. Explain why a soap ion can remove oily dirt from cloth. [2 marks]
- Cue. The hydrophobic tail dissolves in the oil and the hydrophilic head stays in the water, so the dirt is lifted away.
Q3. Hydrolysis of a fat gives of glycerol (). Calculate the moles of glycerol formed. [1 mark]
- Cue. .
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20184 marksSoap is made by the alkaline hydrolysis of fats and oils. (a) Name the two products of this reaction. (b) Explain, in terms of the hydrophilic and hydrophobic parts of a soap ion, how soap removes oily dirt from cloth. (c) State why soap is less effective in hard water.Show worked answer →
Markers reward the two products, a clear hydrophilic-and-hydrophobic mechanism, and the hard-water point.
(a) Alkaline hydrolysis of a fat or oil gives glycerol and the sodium (or potassium) salts of the fatty acids, which are the soap.
(b) A soap ion has a hydrophobic (water-hating) hydrocarbon tail and a hydrophilic (water-loving) ionic head. The hydrophobic tails dissolve into the oily dirt while the hydrophilic heads stay in the surrounding water. With agitation, the oily dirt is surrounded and lifted off the cloth, suspended in a ball-like micelle so it can be rinsed away.
(c) In hard water, soap reacts with dissolved calcium and magnesium ions to form an insoluble scum, so less soap is available to clean.
SQA Higher 20213 marksEmulsifiers are added to many foods. (a) Define an emulsion. (b) Explain how an emulsifier molecule keeps oil and water mixed. (c) State one everyday food that contains an emulsifier.Show worked answer →
A 3 mark answer needs the definition, the mechanism, and an example.
(a) An emulsion is a mixture of small droplets of one liquid suspended in another immiscible liquid, such as oil droplets in water.
(b) An emulsifier molecule has a hydrophilic end that sits in the water and a hydrophobic end that sits in the oil. By bridging the two, it stops the oil and water droplets coalescing and separating into layers, so the emulsion stays stable.
(c) Mayonnaise, salad cream, ice cream or margarine (any one) contains an emulsifier such as lecithin from egg yolk.
Related dot points
- The formation of esters by condensation of an alcohol and a carboxylic acid, the ester link, hydrolysis of esters, and the structure of fats and oils as esters of glycerol and fatty acids, including saturated and unsaturated chains.
An SQA Higher Chemistry answer on esters, fats and oils, covering the condensation reaction that forms an ester from an alcohol and carboxylic acid, the ester link, hydrolysis, and the structure of fats and oils as glycerol esters of saturated and unsaturated fatty acids.
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An SQA Higher Chemistry answer on the oxidation of food, covering oxidation as electron loss, the oxidation of alcohols to aldehydes and carboxylic acids, the rancidity caused by oxidation of double bonds in edible oils, and how antioxidants protect food by being oxidised themselves.
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An SQA Higher Chemistry answer on fragrances and skincare, covering essential oils and terpenes built from isoprene units, the oxidation of terpenes to flavour and aroma compounds, how ultraviolet light damages skin, and how sunblocks and free-radical scavengers provide protection.
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Sources & how we know this
- SQA Higher Chemistry Course Specification — SQA (2018)