What observation is noted during the bromination of alkenes?

During the bromination of alkenes, one of the most significant observations is the decolourisation of bromine water. This reaction occurs because alkenes are unsaturated compounds that can react with bromine, which is a coloured diatomic molecule. When bromine is added to an alkene, the double bond in the alkene undergoes an electrophilic addition reaction with bromine. As the bromine reacts with the alkene, the characteristic reddish-brown colour of bromine water fades, indicating that the bromine is being consumed in the reaction.

This decolourisation serves as a visual test for the presence of double bonds in organic compounds, making it a key observation in identifying alkenes. The other options, while potentially observable in different contexts or reactions, do not specifically correspond to the bromination of alkenes.