What is the difference between an alkane and an alkene?

Alkanes are saturated hydrocarbons: they contain only single carbon to carbon bonds, so every carbon is bonded to as many hydrogen atoms as possible. Alkenes are unsaturated hydrocarbons: they contain at least one carbon to carbon double bond, which is the reactive part of the molecule. This is why alkenes undergo addition reactions and decolourise bromine water, while alkanes do not.

How does the bromine water test distinguish an alkene from an alkane?

Shake the hydrocarbon with orange bromine water. An alkene decolourises the bromine water quickly, turning it from orange to colourless, because the double bond reacts with the bromine in an addition reaction. An alkane does not react in the same way, so the bromine water stays orange. So a rapid orange to colourless change is the positive test for an alkene.

What is the difference between complete and incomplete combustion?

Complete combustion happens when there is plenty of oxygen, and it produces carbon dioxide and water, releasing the most energy. Incomplete combustion happens when oxygen is limited, and it produces carbon monoxide, a toxic gas, and soot (carbon), as well as water, and releases less energy. Incomplete combustion is wasteful and dangerous, which is why fuel appliances need a good air supply.

How is ethanol made by fermentation and what is it used for?

Ethanol is made by fermentation when yeast acts on a sugar solution at a warm temperature in the absence of air. The enzymes in the yeast convert the sugar into ethanol and carbon dioxide. Ethanol is widely used as a solvent, as a fuel (sometimes blended with petrol), and in alcoholic drinks. It is a member of the alcohols, the family whose members contain the characteristic hydroxyl group.

How is Organic Chemistry assessed in N(A)-Level Science 5107?

It is examined in the two Chemistry papers. Paper 3 (multiple choice, 20 marks) tests recognising alkanes and alkenes, the bromine water test, and the products of combustion. Paper 4 (structured, 30 marks) asks you to write a combustion equation, compare complete and incomplete combustion, describe the bromine water test with its result, and describe how ethanol is made by fermentation. There is no separate practical paper.

SingaporeChemistry

Organic Chemistry (Singapore N(A)-Level Science Chemistry 5107): the alkanes as fuels, the alkenes and the bromine water test, and ethanol as an alcohol with its uses

A Singapore N(A)-Level Science Chemistry (SEAB 5107) overview of Organic Chemistry. The alkanes as saturated hydrocarbon fuels and their complete and incomplete combustion, the alkenes as unsaturated hydrocarbons distinguished by the bromine water test, and ethanol as a member of the alcohols made by fermentation, with links to every dot point.

Generated by Claude Opus 4.85 min readSEAB-5107Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is really about
Alkanes and fuels
Alkenes and addition reactions
Alcohols and their uses
How this topic is examined
Check your knowledge

What this topic is really about

Organic Chemistry in N(A) Chemistry is a first look at carbon compounds, focused on three small families: the alkanes, the alkenes and the alcohols. The big ideas are that carbon forms chains, that a family of compounds shares a pattern, and that a reactive feature such as a double bond or a hydroxyl group decides how a compound behaves. This guide ties the three dot points together and links to each one for the worked answers and practice.

The complete set of dot-point pages for this topic lives at /sg-n-level/chemistry/syllabus/organic-chemistry.

Alkanes and fuels

Alkanes and fuels introduces the alkanes as saturated hydrocarbons, with only single carbon to carbon bonds, and explains why they are such good fuels. The key comparison is complete versus incomplete combustion: complete combustion in plenty of oxygen gives carbon dioxide and water, while incomplete combustion in limited oxygen gives the toxic gas carbon monoxide and soot.

Alkenes and addition reactions

Alkenes and addition reactions covers the alkenes as unsaturated hydrocarbons with a carbon to carbon double bond. That double bond is reactive, so alkenes undergo addition reactions and decolourise bromine water, which is the standard test that tells them apart from alkanes.

Alcohols and their uses

Alcohols and their uses introduces ethanol as a member of the alcohols, describes how it is made by the fermentation of sugar by yeast, and lists its main uses as a solvent, a fuel and the alcohol in drinks.

How this topic is examined

Balance combustion equations. Practise complete combustion of small alkanes, and state carbon dioxide and water as the products.
Compare complete and incomplete combustion. Name the products of each and note that incomplete combustion makes toxic carbon monoxide and releases less energy.
State the bromine water test fully. Orange to colourless for an alkene, no change for an alkane.

Check your knowledge

A mix of recall, reasoning and equation questions covering Organic Chemistry. Attempt them under timed conditions, then check against the solutions.

State the difference between a saturated and an unsaturated hydrocarbon. (2 marks)
Write the balanced equation for the complete combustion of methane. (1 mark)
State the products of the incomplete combustion of a hydrocarbon and one danger of incomplete combustion. (3 marks)
Describe the bromine water test and its result for an alkene. (2 marks)
Describe how ethanol is made by fermentation. (3 marks)

Solutions

Q1: A saturated hydrocarbon (an alkane) has only single carbon to carbon bonds. An unsaturated hydrocarbon (an alkene) has at least one carbon to carbon double bond.
Q2: $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$ .
Q3: Incomplete combustion produces carbon monoxide, soot (carbon) and water. A danger is that carbon monoxide is a toxic gas (it can also release less energy and produce soot that clogs appliances).
Q4: Add bromine water to the alkene and shake. The bromine water changes from orange to colourless, showing the alkene is unsaturated.
Q5: Add yeast to a warm sugar solution and leave it in the absence of air. Enzymes in the yeast convert the sugar into ethanol and carbon dioxide. The ethanol can then be separated by fractional distillation.

Sources & how we know this

Singapore-Cambridge GCE Normal (Academic) Level Science (Physics, Chemistry) and Science (Chemistry, Biology) (Syllabus 5107) — Singapore Examinations and Assessment Board (2026)
Cambridge Assessment International Education, working with SEAB on the Singapore-Cambridge GCE N(A)-Level — Cambridge Assessment International Education (2026)