Practice Questions

1.Name the chemical solution used to test for starch.

Iodine solution

2.What colour change indicates a positive starch test?

Blue-black colour forms

3.Give two reasons why a leaf is boiled in water before testing for starch.

Boiling the leaf in water kills it. This stops all enzyme activity, including enzymes that could break down starch. Without this step, starch might be broken down before you test it, giving a false negative.

Boiling also softens the leaf tissue, making it easier for ethanol to penetrate  the leaf in the next step.

4.Explain why only the green parts of a variegated leaf turn blue-black in a starch test.

The green parts of the leaf contain chlorophyll, these are the parts of the leaf that carry out photosynthesis.