What makes a molecule an alcohol, and how are alcohols named and used?
Alcohols: the hydroxyl functional group, naming the straight-chain alcohols, their properties and their uses as solvents and fuels.
An SQA National 5 Chemistry answer on alcohols, covering the hydroxyl functional group, naming the straight-chain alcohols with the -ol ending, their properties such as solubility and flammability, and their uses as solvents and fuels.
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What this key area is asking
The SQA wants you to recognise the hydroxyl functional group, name the straight-chain alcohols using the -ol ending, describe their key properties, and state their main uses as solvents and fuels. Alcohols are the first of the functional-group families in Nature's Chemistry, and they link forward to carboxylic acids.
The functional group
Every alcohol contains this -OH group, and it is the reason all alcohols share similar chemistry.
Naming the alcohols
Properties of alcohols
The hydroxyl group controls the properties:
- The lower alcohols (methanol, ethanol, propanol) mix easily with water, because the -OH group attracts water molecules strongly. Solubility falls as the carbon chain gets longer.
- Alcohols have higher boiling points than alkanes of similar size, again because of the attraction between the -OH groups.
- Alcohols are flammable and burn to release energy.
Uses of alcohols
Worked example: naming from the carbon number
Alcohols as part of a homologous series
Alcohols form a homologous series, just like the alkanes. Each member differs from the next by a unit, they share the same general behaviour because they share the -OH group, and their physical properties change gradually. As the carbon chain gets longer, the boiling point rises and the solubility in water falls, because the bigger hydrocarbon part of the molecule has less in common with water. This is why methanol and ethanol mix with water in any proportion, but longer-chain alcohols mix less readily. Recognising alcohols as a series helps you predict the name, formula and rough properties of a member you have not met before.
Examples in context
Ethanol is the alcohol that touches daily life most: it is the active component of hand sanitiser, where it acts as a solvent and disinfectant; it is blended into petrol as bioethanol, made by fermenting sugars, to reduce reliance on fossil fuels; and it is the alcohol in drinks. Methanol and propanol are common laboratory and industrial solvents. The shared hydroxyl group is what makes all of them mix with water and burn as fuels. The fact that alcohols burn cleanly, releasing carbon dioxide and water, is why a spirit burner in the school lab usually runs on ethanol or methanol.
Try this
Q1. Name the functional group found in all alcohols. [1 mark]
- Cue. The hydroxyl group, -OH.
Q2. Name the alcohol that has two carbon atoms. [1 mark]
- Cue. Ethanol.
Q3. State two everyday uses of alcohols. [2 marks]
- Cue. As a solvent (cleaning products, cosmetics) and as a fuel (ethanol in spirit burners or petrol).
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 N5 2018 style3 marksName the functional group present in all alcohols, name the alcohol with three carbon atoms, and state one use of alcohols.Show worked answer →
Markers reward the functional group, the correct name and a valid use.
The functional group in all alcohols is the hydroxyl group, written as -OH. It is the group that gives alcohols their characteristic properties.
The alcohol with three carbon atoms is propanol; alcohols are named from the matching alkane with the ending changed to -ol, so propane becomes propanol.
A valid use is as a solvent (for example in cleaning products or cosmetics) or as a fuel (for example ethanol burned in a spirit burner or blended into petrol). Either correct use earns the mark.
SQA N5 2020 style3 marksEthanol is widely used as a fuel. State the products of the complete combustion of ethanol, and explain why ethanol mixes easily with water.Show worked answer →
A 3 mark answer needs the two combustion products and a reason for the solubility.
The complete combustion of ethanol, like any carbon compound burned in plenty of oxygen, produces carbon dioxide and water, releasing energy.
Ethanol mixes easily with water because of its hydroxyl (-OH) group. This group lets the small alcohol molecule attract water molecules strongly, so the lower alcohols such as ethanol are very soluble in water. This is one reason ethanol is so useful as a solvent.
Related dot points
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An SQA National 5 Chemistry answer on carboxylic acids, covering the carboxyl functional group, naming the straight-chain acids with the -oic acid ending, their reactions with bases to form salts and water, and everyday examples such as ethanoic acid in vinegar.
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Sources & how we know this
- SQA National 5 Chemistry Course Specification — SQA (2019)