# How does bromine water show if something is an alkane or alkene?

You know that the bromine molecule is polarizable, i.e. it can form $B r - B {r}^{\delta +}$ (the atom on the LHS is ${\delta}^{-}$!). This electrophilic centre is capable of reaction with the electron-rich double bond to form a cationic intermediate, i.e. with ethylene:
${H}_{2} C = C {H}_{2} + B {r}_{2} \rightarrow {H}_{2}^{+} C - C {H}_{2} B r + B {r}^{-}$
The orange colour of bromine should therefore dissipate, and of course this is a positive test for olefinic bonds. Now that you have a carbocation, this will react with any nucleophile present: this could be water to give $H O C {H}_{2} - C {H}_{2} B r$ (and of course water is a potent nucleophile). In the absence of water in an inert solvent it could be the bromide ion that was delivered in the first part of the bromination, to give 1,2-dibromoethane.