Two important homologous series to know in IGCSE Chemistry are alkanes and alkenes.

Alkane

• A homologous series of hydrocarbons that contains only single carbon bonds.
• Since alkanes contain only single carbon bonds, they are saturated hydrocarbons.
• The general formula for alkanes are C_nH_2n+2, where n is the number of carbons in the alkane.

Butane and pentane have isomers: Isomers are compounds that have the same molecular formula but have different structures.

Reaction of alkanes with halogens:

• Substitution reaction is a reaction in which one atom is swapped with another atom.
• In the presence of UV light, alkanes undergo a substitution reaction with halogens.

Alkene

• A homologous series of hydrocarbons that contains double carbon bonds.
• Since alkenes contain double carbon bonds, they are unsaturated hydrocarbons.
• The general formula for alkenes are C_nH_2n, where n is the number of carbons in the alkene.

Butene has two isomers based on the location of the carbon double bond.

Reaction of alkenes with bromine:

• Alkenes undergo an addition reaction with bromine.
• Diatomic bromine is added across the carbon double bond of the alkene.

Using Bromine Water to Distinguish Alkanes and Alkenes

• Bromine water is an orange-coloured solution of bromine.
• When bromine water is shaken with an alkane, there is no change.
• When bromine water is shaken with an alkene, bromine water becomes decolourised as the addition reaction to the double carbon bond results in the formation of a colourless dibromoalkane.

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