Bromothymol blue is a pH indicator that changes colour depending on the acidity of a solution. Table 1 shows the colour of bromothymol blue at different pH ranges.

Table 1

A student investigated the effect of exercise on human respiration. They bubbled their exhaled air at a constant rate through a tube containing a blue solution of bromothymol blue (initially at pH 7.8).

• Before exercise, it took 52 seconds of bubbling for the solution to turn yellow.
• Immediately after exercise, it took only 18 seconds of bubbling for the solution to turn yellow.