A leaf was plucked from a plant on a sunny day and kept for 2 minutes in boiling water. It was subsequently immersed in boiling alcohol and treated with iodine solution. What will be the final colour of the leaf after the test?
- (a)Colourless
- (b)Green
- (c)Blue
- (d)White
Correct — C, Blue. The leaf was picked on a sunny day, so its cells are loaded with the starch photosynthesis has been laying down all morning. Everything that follows is preparation rather than test. Two minutes in boiling water kills the cells and softens the membranes so reagents can get in. Boiling alcohol then dissolves the chlorophyll away and leaves a pale, almost white sheet. Only now does iodine reach the starch, and starch with iodine gives the familiar blue-black — the amylose in starch coils into a helix that traps triiodide ions, and that complex is the colour you see. Of the four printed choices, blue is the one describing the finished test.
- (a)Colourless — That is the leaf one step earlier, after the alcohol has taken the chlorophyll out but before iodine has been added. Adding iodine is precisely what changes it.
- (b)Green — The green has already gone by the time iodine arrives. Chlorophyll dissolves in hot alcohol, and removing it is the whole reason that step exists — a green leaf would hide any colour the iodine produced.
- (d)White — Another description of the intermediate stage. The decolourised leaf is a pale off-white sheet, and that pallor is the background against which the iodine colour is meant to show.
The iodine test is the standard school demonstration that a green leaf in sunlight makes starch. Glucose from photosynthesis is stored in the leaf as starch, so starch in the leaf is taken as evidence that photosynthesis has occurred. Iodine is specific to it: a dilute iodine solution turns starch blue-black but leaves cellulose, protein and fat unmarked. The three preparatory steps — hot water, hot alcohol, then iodine — each remove one obstacle between the reagent and the starch.
The trap is to answer with whatever the leaf looks like after a step you happen to remember, and three of the four options describe exactly that. Read the sequence to its end: the last thing done to the leaf is the iodine, so the last colour is iodine's. Two teaching points sit alongside. The alcohol is warmed in a water bath rather than over a flame, because alcohol catches fire easily. And the control that makes the experiment mean anything is a plant kept in the dark for a day or two beforehand — its destarched leaf gives only the pale brown of the iodine itself. One printed nicety: the textbook result of the test is usually written blue-black, and the paper's shorter 'Blue' is the same colour named loosely.
- Iodine gives starch a blue-black colour; the amylose helix traps triiodide ions to form the coloured complex.
- Chlorophyll dissolves in hot alcohol but not in water, which is why the decolourising step uses alcohol.
- The alcohol is heated in a water bath and not over a direct flame, because it is inflammable.
- Cellulose is also a glucose polymer, yet gives no colour with iodine — the test picks out starch specifically.
Three of the four options describe a stage before the test is finished. The answer is whatever the last reagent does.
- Answering with the colour after the alcohol step instead of after the iodine.
- Forgetting that the leaf must be decolourised first, and expecting green to persist.
- Reading the boiling-water step as part of the test rather than as a way of making the tissue permeable.
Either as this stepwise item asking for the final colour, or as a question on why one particular step — the alcohol, the water bath, the dark treatment — is carried out.
Which one of the following statements regarding starch and cellulose is NOT correct?
- (a) Both of them are of plant origin
- (b) Both of them are polymers
- (c) Both of them give colour with iodine
- (d) Both of them are made up of glucose molecules
Answer(c) Both of them give colour with iodine
The chemistry underneath this leaf experiment. Iodine picks out starch and ignores cellulose, which is exactly why a stained leaf proves starch is present rather than merely that plant tissue is present.
- practice — not a real PYQ
In the starch test on a leaf, the leaf is boiled in alcohol mainly in order to
- (a)kill the cells of the leaf
- (b)remove the chlorophyll
- (c)dissolve the starch
- (d)make the leaf permeable to iodine
Answer(b) remove the chlorophyll — the green pigment dissolves in hot alcohol, leaving a pale leaf on which the iodine colour can be seen.
- practice — not a real PYQ
A potted plant is kept in complete darkness for two days and a leaf is then tested with iodine solution. The most likely result is that the leaf
- (a)turns blue-black
- (b)turns bright red
- (c)shows only the brown colour of the iodine
- (d)turns deep green
Answer(c) shows only the brown colour of the iodine — darkness stops photosynthesis, so the leaf has been destarched and there is nothing for the iodine to stain.