why leaf is diped in hot water and then alcohol and then to iodine.. ?can we dip staright to iodine solution
The expeiment is conducted to test the presence of starch in the leaf. In this experimentthe leaf is dipped in the hot water for about one minute so as to get rid off the waterproof layer so that the leaf becomes soft. Then this leaf is soaked in boiling alcohol in a water bath to remove chlorophyll. Finally iodine is added to test the presence of starch.
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Heating the leaf in alcohol removes all pigment from it. If we try to test for starch without doing this, the colour change due to the starch indicator(iodine) will get masked.
The leaf is dipped in hot water so as to kill the cells of the leaf, so that no activities take place inside the cells. It is dipped in alcohol to bleach the leaf (remove the green colour). On doing this the leaf becomes pale white in colour and the alcohol in the beaker becomes green in colour due to the chlorophyll pigments. Any change in colour can be easily observed against a white background, when a few drops of Iodine is placed on the leaf.
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