Question

why do we boil the leaf in alcohol when we are testing it for starch

Answer 1

When a leaf is boiled in alcohol, two things happen.

Firstly, the leaf gets killed under heat and secondly extraction of chlorophyll becomes easy when ethyl alcohol reaches inside the leaf. What we have in hand now is a dead leaf with no green color, all set to be tested for starch. 

Since the presence of starch can only be deduced by iodine, we place this leaf in the iodine solution. If the leaf turns blue black it is indicative of the presence of starch and if it turns reddish brown it means that the leaf does not have starch.

Answer 2

The leaf is boiled to remove its chlorophyll. Green colour of chlorophyll might hinder the reaction of starch with iodine

I hope this answer will help to u.....