Question

why mercury is put in bromine water

Answer 1

Hey There !!

Because Mercury has a special electron configuration that means the bonds between the mercuryatoms are much weaker than the bonds of other metals, so it's liquid at room temperature instead of solid. Bromine just happens to have a boiling point above room temperature - it's not unusual for its group, or anything.

Answer 2

Mercury on heating with bromine gives mercuric bromide, HgBr2.

                Hg + Br2 = HgBr2

It is a simple combination reaction between the two liquid elements.

(Mercuric bromide on heating with more mercury gives mercurous bromide, Hg2Br2)