When ethene (C₂H₄) is mixed with bromine water (Br₂ dissolved in water), a chemical reaction takes place. This reaction is an example of an addition reaction, where the double bond in ethene reacts with the bromine molecules, causing the bromine atoms to add to the carbon atoms of the double bond.

The reaction can be represented as follows:

C₂H₄ + Br₂ → C₂H₄Br₂

In this reaction, the double bond between the two carbon atoms in ethene is broken, and each carbon atom forms a bond with a bromine atom. The product of the reaction is 1,2-dibromoethane (C₂H₄Br₂), which is a colorless liquid with a sweet odor.

The reaction between ethene and bromine water is a rapid and exothermic process, meaning that it releases heat. The bromine water turns from its original orange color to a colorless solution as the reaction proceeds. This color change can be used as an indicator to detect the presence of double bonds in unsaturated hydrocarbons like ethene.