Why does bromine discolor water

Bromine water

Author: Hans Lohninger

Bromine water, Br2(aq), is an orange-brown solution of elemental bromine in water. Bromine dissolves in water with a maximum of 0.2 mol / l, the solution reacts slightly acidic, as there is a disproportionation of bromine in hydrogen bromide and hypobromous acid:

Br2 + H2O HBr + HBrO

Since this reaction is accelerated by light, bromine water must be stored in dark or opaque bottles.

To make bromine water, you fill a bottle halfway with distilled water and let the bromine vapor from a bottle with elemental (liquid) bromine flow into the half-empty bottle (bromine vapor is heavier than air). After shaking the water, the bromine vapor dissolves and the solution takes on an orange to brown color. To increase the concentration, you can repeat the transfer of the bromine vapor. By repeating it three times, an approximately 0.1 molar solution is obtained.

Bromine water is used in analysis for the quick and easy detection of C-C multiple bonds. If you shake bromine water with the substance to be tested (assuming that the substance is liquid), the bromine water quickly becomes discolored if at least one C-C double bond is present. The discoloration is due to the electrophilic addition of bromine to the double bond; the resulting dibromine compound is usually colorless.