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When carrying out the starch test on a leaf, why is it important to boil the leaf in alcohol?
(a) to dissolve the waxy cuticle (b) to make the cells more permeable to iodine solution
(c) to remove the chlorophyll
(d) to stop chemical reactions in the cells.>
Correct Answer: (c) To remove the chlorophyll
Explanation: A green leaf is bleached before carrying out a starch test by boiling the leaves in alcohol. It breaks the cell wall of the leaf and removes the chlorophyll present in the leaves. Chlorophyll is removed so that the leaf can absorb iodine solution and turn into a blue-black color. Thus, we can properly identify the starch present in the leaf.
Test for starch in plants:
1. Keep one of the plants in a dark room and another one on a sunny windowsill for 24 hours.
2. Fill the beaker or jar with ethyl alcohol. Keep it in a saucepan full of water. Heat the pan until the ethyl alcohol begins to boil.
3. Dip each of the leaves in the hot water for 60 seconds and then in the ethyl alcohol for 2 minutes, or until they turn almost white. Keep them aside in a separate shallow dish.
4. Cover the leaves with iodine solution and observe.
Reaction: The hot water kills the leaf, and the alcohol breaks down the chlorophyll. This removes the green color of the leaf. When we put iodine on the leaves one of them will turn blue-black and the other will be reddish-brown. Because the leaf that was in the light performed photosynthesis and produced starch. Iodine is an indicator that turns blue-black in the presence of starch.
Precaution: Keep ethanol away from naked flames. Wear eye protection while working with ethanol and iodine solution.
Updated on: 27-Mar-2023
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