Bromine water is brown and weakly acidic due to following equilibrium :
` underset( " Brown " ) (Br_2(aq) ) + 2H_2O hArr underset( " Colourless ")( HBrO(aq))+ H_3O^(+) (l) + underset( " Colourless ") (Br^(-)(aq)) `
When sodium hydroxide is added to the solution , the solution becomes colourless but the colour return when hydrochloric acid is added. Explain this observation.

Explanation

On adding sodium hydroxide, `OH^(-)` ions from NaOH combine with `H_(3)O^(+)` ions present at equilibrium to form unionised water. The removal of `H_(3)O^(+)` ions from the solution causes the equilibrium to shift in the forward direction. Therefore, brown coloured `Br_(2)` molecules change into colourless ions. Hence, the solution becomes colourless. In other way, addition of HCl supplies `H_(3)O^(+)` ions. The increase in the concentration of `H_(3)O^(+)` ions causes the equilibrium to shift in backward direction resulting in the formation of coloured bromine molecules. Hence, the colour of the solution returns.