Photosynthesis consumes carbon dioxide, which changes the pH of a solution. This interactive illustrates how photosynthesis can be followed by the color change of the pH indicator Bromothymol blue (BTB). Instructions: To follow photosynthesis over time, drag the slider to see how the pH of the BTB solution containing the water plant Elodea changes over time. Check the boxes to see how oxygen and carbon dioxide levels change during photosynthesis.