Unsaturated – IGCSE Chemistry Definition

The word “unsaturated” is tested both as a definition and through the practical bromine water test. Understanding that unsaturation means a C=C double bond is present — and that this bond opens in addition reactions — is crucial for scoring marks across the organic chemistry topic.

The 0620 definition

An unsaturated compound contains at least one carbon–carbon double bond (C=C).

The carbon atoms involved in the double bond are not bonded to the maximum possible number of other atoms — they could bond to more if the double bond opened.

Unsaturated hydrocarbons: alkenes

Alkenes are the unsaturated homologous series. Their general formula is CₙH₂ₙ.

Compare: ethane (C₂H₆, saturated) vs ethene (C₂H₄, unsaturated). Ethene has two fewer hydrogen atoms because two C−H bonds are replaced by the C=C bond.

The bromine water test

This is the definitive test for unsaturation:

1. Add a few drops of bromine water (orange-brown) to the compound
2. Shake

Result	Meaning
Bromine water decolourised	Unsaturated — C=C bond present
Bromine water stays orange-brown	Saturated — no C=C bond

The bromine adds across the double bond in an addition reaction:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

Reactions of unsaturated compounds

Because of the C=C double bond, unsaturated compounds undergo:

• Addition reactions with Br₂, H₂, H₂O, HBr
• Addition polymerisation to form polymers

They also undergo combustion, like all hydrocarbons.

Worked exam question

A student has two colourless liquids: hexane and hex-1-ene. Describe a test to distinguish between them. [2]

Mark scheme

Add bromine water to each [1]; hex-1-ene decolourises the bromine water (it is unsaturated / has a C=C bond), hexane does not (it is saturated) [1]

Common exam mistakes

• Saying “decolourises” when you mean “turns colourless” — actually, “decolourises” is correct. But saying “turns clear” is wrong. Clear means transparent; colourless means no colour.
• Confusing unsaturated fats (biology/nutrition) with unsaturated compounds in chemistry — the principle is the same (C=C bonds present), but the exam context is organic molecules.
• Stating that all unsaturated compounds are alkenes — other compounds can be unsaturated too (e.g. carboxylic acids are not unsaturated in the C=C sense).