Saturation Test (Edexcel International A Level (IAL) Chemistry): Revision Note

Saturation Test

• Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond)

• Br₂(aq) is an orange or yellow solution, called bromine water and this is the halogen most commonly used

• The unknown compound is shaken with the bromine water

• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

  • The orange bromine water turns colourless at room temperature, for example CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane)

• A saturated compound such as an alkane does not decolourise bromine water in the dark, so the orange colour stays

  • Alkanes only react with bromine in UV light, by free-radical substitution, so the test is done without UV light

The decolourisation of bromine water by an unsaturated compound as a result of an addition reaction