In the test for starch, the leaf is first placed in boiling water to

The correct answer is "kill and make the leaf permeable". Placing the leaf in boiling water helps to kill the cells of the leaf and make the cell membrane more permeable, allowing the iodine solution used in the test to enter the cells and react with any starch present. The heat also denatures the enzymes that might break down the starch, which could otherwise interfere with the test. The boiling water doesn't have any effect on the chlorophyll or the waxy cuticle of the leaf, and it doesn't directly cause the leaf to turn blue-black; rather, it provides the necessary conditions for the iodine solution to do so.