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 addition of NaOH, the `OH^(-) ` ions given by NaOH combine with `H_3^(+)` ions present at equilibrium to form unionised water. The removal of `H_3O^(+)` ions from the solution causes the equilibrium to shift in the forward direction. Therefore, brown colouured `Br_2` molecules change into colourless `Br^(-)` ions. Hence the solution becomes colourless. On the other hand, addition of HCl furnishes` H_3O^(+) ` ions. The increase in the concentration of `H_3O^(+) ` ions causes the equilibrium to shift in the backward direction resulting in the formation of coloured bromine molecules. Hence, the colour of the solution returns back.