To determine when starch digestion was complete, you would look for the solution that no longer turned blue-black. This indicates that the starch has been broken down into simpler sugars that do not react with iodine, which is the chemical used to test for starch.
When starch is present, it reacts with iodine to form a blue-black color. If the solution turns pink or purple, it suggests the presence of other substances, such as certain sugars, but does not indicate the complete digestion of starch. Therefore, the correct answer is:
- C) The solution no longer turned blue-black
This observation shows that the starch has been fully digested and is no longer detectable in the solution.