A leaf is detached from a tree and tested with iodine. The leaf turns dark blue.

   (a) What does this result tell you?

   (b) Why is this result not sufficient evidence to confirm that photosynthesis had taken place in the leaf?

a) If a (suitably prepared) leaf goes blue with iodine, it tells you that starch is present.

b) Unless the leaf is known to have been free from starch at the beginning of the experiment you cannot conclude that photosynthesis has occurred. The starch may be permanently there in this leaf. Also, in the absence of an experimental design (with a control), there is no telling where the starch has come from. It may have been produced from sucrose  which was transported to the leaf from another part of the plant.