Predict what would happen if you used the pH indicator thymophthalein instead of phenol red in the above setup. Thymophthalein solution appears colorless below pH 9.3, but blue at higher pH.

Question

What will be an ideal response?

Answer 1

Phenol red turns yellow when exposed to acid. There would be no color change unless the acidity of the solution increased above 9.3. If the pH increased above 9.3, the solution would turn blue. As the Elodea stems are placed into this solution, the color would have disappeared as the acid was “picked up”.