Question 12
Amylase is mixed with a starch solution. After 20 minutes, a drop of the mixture no longer turns blue-black with iodine. Why?
The right answer is the one marked above.
Because there is no starch left. Amylase has acted on it and turned it into maltose, and maltose does not give the iodine colour.
Sampling every two minutes shows the change happening: the colour goes from blue-black, gradually through to brown — the colour of the iodine solution itself — as the last of the starch disappears. This is what "an enzyme catalyses a reaction" looks like on a white tile.