Question 3
After the starch test, leaf D shows no colour change and leaf C turns dark purple. What does this show?
The right answer is the one marked above.
Carbon dioxide is necessary for photosynthesis. Both leaves had the same light, the same water and the same few hours. Only the carbon dioxide differed.
Note that nobody added extra carbon dioxide to leaf C. The experiment works by subtraction only — take one thing away and see what stops.