Sara placed a healthy potted plant in a dark room for 48 hours to perform an experiment on photosynthesis. She plucked one of the leaves and tested it for starch. The leaf did not turn blue-black on adding Iodine solution.
What is the significance of boiling the leaf in alcohol during the starch test?
Topic: Starch test — boiling in alcohol
Answer
Boiling in alcohol REMOVES THE CHLOROPHYLL and decolourises the leaf, so that the blue-black colour given by iodine can be seen clearly against the pale background.
A green leaf would hide the colour change entirely — blue-black on green is impossible to read. Removing the pigment makes the result visible.
THE FULL SEQUENCE of the starch test, with the reason for each step, since any of them may be asked:
- Boil the leaf in WATER — to kill the cells and stop all chemical activity, and to soften the leaf.
- Boil it in ALCOHOL (methylated spirit) — to dissolve out the chlorophyll.
- Dip it in WARM WATER — to soften the leaf again, since alcohol makes it brittle.
- Add IODINE solution — blue-black means starch is present, brown means it is absent.
ONE SAFETY POINT examiners like: alcohol is inflammable, so the leaf is heated in a WATER BATH and never directly over a flame.