Alkanes do not decolourise aqueous bromine. This shows that they are. hydrocarbons

Alkanes are indeed hydrocarbons, specifically they are saturated hydrocarbons. This means they contain only single bonds between the carbon atoms in their molecular structure. The fact that they do not decolourise aqueous bromine is because they do not undergo addition reactions, which is characteristic of unsaturated hydrocarbons like alkenes and alkynes.

In an addition reaction, the bromine molecule (Br2) would add across the carbon-carbon double bond of an alkene, breaking the double bond and forming two new carbon-bromine single bonds. This reaction would use up the bromine, causing the reddish-brown colour of the bromine water to disappear.

However, alkanes do not have carbon-carbon double bonds, so they do not react with bromine in this way. Therefore, the bromine water remains its original colour, indicating that no reaction has taken place. This is a simple test to distinguish between saturated and unsaturated hydrocarbons.