Bromine water goes colorless because bromine molecules react with the alkene's carbon-carbon double bond and are consumed. This electrophilic addition reaction produces a colorless dibromo compound in the standard IB model.
Bromine water is orange-brown because it contains dissolved bromine, . An alkene contains a region of high electron density in its carbon-carbon double bond. As a bromine molecule approaches, the electrons in the double bond repel its electrons and induce a temporary dipole.
The electron-rich double bond attracts the partially positive end of the polarized bromine molecule. During the reaction, the weaker component of the double bond breaks, and bromine atoms are added to the two carbon atoms. A carbon-carbon single bond remains, while two new carbon-bromine bonds form.
Because colored is converted into a colorless organic product, the orange-brown color disappears. For ethene, the simplified equation is: