Why did the solution that contained Elodea maintained in light change from pale yellow to red?

When the sprig of Elodea was added to the solution and placed in the light, the Elodea continued to photosynthesize. As a result, carbon dioxide was removed from the solution. Just as the addition of carbon dioxide to a solution increases proton concentration (and therefore, decreases pH), the removal of carbon dioxide from a solution decreases proton concentration (and therefore, increases pH).The phenol red solution is yellow when the pH of the solution is less than 7 and turns red as carbon dioxide is consumed and the pH of the solution rises to 7 or greater.

Did the solution that contained Elodea maintained in dark change from pale yellow to red? Why?

The solution containing Elodea maintained in the dark did not undergo a color change as seen in the Elodea in the light. Although the second phase of photosynthesis (which consumes carbon dioxide, resulting in the pH change which alters the color of the solution) does not require light, it does require the ATP and NADPH formed in the light-dependent reactions. The amount of carbon dioxide consumed by the Elodea maintained in the dark, and therefore, the color of the phenol solution, will depend on its ATP and NADPH stores.