Why does water containing bromothymol blue turns yellow when carbon dioxide is bubbled into it?
Bromothymol blue is a pH indicator. It changes colour depending on the pH conditions in the liquid. This indicator is yellow when pH is below 6.0. It is blue when the pH of the solution is above 7.6 So if you add bromothymol blue to water that has pH above 6.0, it will have a blue/greeen colour. When you bubble carbon dioxide into the water, it forme an acidic solution. This reduces the pH to below 6.0, and so the indicator turns yellow. A more technical explanation is that bromothymol blue is a weak acid. Let us call this HB for short. In water this acid is able to dissociate: HB ↔ H+ + B- The B- ion has a blue colour, and that is what you see in a solution with pH> 7.6. If you introduce an acid into the solution, this causes the equilibrium of the HB dissociation to move to the left. There is no dissociated HB left, and no blue coloured B- ion. You have a solution containing the undissociated acid. The colur of this acid solution is yellow, which is what you see when the pH<6.0.
