Question

Why do we boil leaf in alcohol when we are checking it for starch?

Answer 1

Hint: Alcohol is an organic solvent which has a wide range of non-polar compounds.

Complete answer:
We boil the leaf in alcohol when we are checking it for starch to eradicate chlorophyll, which is the green pigment present in leaves. During the starch test, we observe the colour change of leaf from brown to blue when iodine is put on the leaf as iodine reacts with the starch present in the leaf. But, primarily the leaf is green in colour which does not allow the required observation to appear. Hence to dissolve the chlorophyll or the green pigment present in the leaf we boil the leaf in alcohol when we are testing it for starch.

Additional Information: In the leaves, excess glucose is rapidly transformed into starch, so we test leaves for starch to show that photosynthesis has occurred in the leaves, rather than checking for glucose. This experiment allows us to prove that photosynthesis occurs in the presence of sunlight, water and carbon dioxide.

Note: The cells are enclosed by cell membranes and thick cellulose cell walls and some leaves also have a protective waxy cuticle. The hot water treatment is used to weaken up these protective structures and break the cell membranes to let the chlorophyll out and the iodine solution in so that we can see the required colour changes.