What happens to the bromine water when it is mixed with an alkene?

When bromine water is mixed with an alkene, the solution turns colorless due to a chemical reaction known as electrophilic addition. Alkenes are unsaturated hydrocarbons that contain a carbon-carbon double bond. The presence of this double bond makes alkenes reactive, particularly with halogens such as bromine.

In this reaction, bromine molecules (Br2) react with the double bond in the alkene, resulting in the opening of that double bond. As the bromine atoms add across the double bond, they form a dibromoalkane, which is typically colorless in solution. This decolorization of the bromine water is a key indicator of the presence of an alkene, as saturated compounds like alkanes do not undergo this reaction and, therefore, do not change the color of bromine water.

This understanding is crucial in organic chemistry, as it not only demonstrates the reactivity of alkenes but also serves as a qualitative test to distinguish between saturated and unsaturated hydrocarbons.