We boil the leaf in alcohol when we are testing it for starch because to remove the green pigment chlorophyll present in it. For the starch test, we need to observe the colour change from brown to blue when iodine is put on the leaf. But, initially the leaf is green in colour which does not show the required observation. Hence to remove the green pigment present in the leaf we boil the leaf in alcohol when we are testing it for starch.