Question

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

Answer 1

Because Methylated spirit is analcohol & when we boil the leaf in alcohol we are testing it for starch as this would remove the chlorophyll from the green leaf . Deolourish leaf is than treated with iodine & a blue black color is obtained which confirms the presence of starch in the green portion of leaf.
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Answer 2

your answer is above