Photosynthesis · Investigation
Testing a leaf for starch
Four preparation steps before a single drop of iodine touches the leaf. Every one of them exists because somebody once left it out and got a result they could not read.
Start here
Put iodine straight onto a green leaf and you learn nothing.
The leaf is green, the iodine is orange-brown, and whatever happens underneath is invisible. It is also waterproof on both surfaces and its cells are alive and still working. Three problems, and each one has its own step in the method.
Why is the leaf boiled in ethanol before the iodine goes on?
To dissolve the chlorophyll out, so the leaf goes pale and any colour change can actually be seen. Notice what that means: by the time the iodine arrives, the chlorophyll has already been poured down the sink. Whatever turns blue-black, it is not that.
At the bench · leave a step out and see
Run the test badly on purpose
not run yet
One leaf from a plant that has been in bright light all day, half of it covered with foil. Switch any step off, choose how you heat the ethanol, then add the iodine and read what you get.
- Destarch the plantTwo days in a dark cupboard before the experiment begins.
- Boil the leaf in waterAbout a minute in a beaker of boiling water.
- Boil in ethanolIn a tube of ethanol, stood in hot water.
- How the ethanol is heatedThe choice that matters more than any other on this bench.
- Dip in hot waterA few seconds, then spread flat on a white tile.
Result unreliable
Patchy, faint, and different every time.
The cells were never killed or opened up, so the iodine soaks in unevenly through a leaf that is still waxy and intact. Some patches go dark, some do not, and repeating it gives a different pattern.
Can you conclude anything? Not safely. You cannot tell whether a pale patch means no starch or just no iodine — which is exactly the ambiguity the method exists to remove.
No control
Blue-black everywhere — including under the foil.
The covered half went black too. The plant had been making starch for days before you covered anything, and that starch is still sitting in every leaf. Your foil changed today; the starch is older than that.
Can you conclude anything? Nothing about light. The result is real and it is unusable, because you cannot tell when the starch was made. Destarch for two days and start again.
Result unreadable
A dark green leaf with iodine on it.
The chlorophyll is still in the leaf, so the leaf is green, the iodine is orange-brown, and any blue-black underneath is invisible against it. You have a stained green leaf and no information.
Can you conclude anything? Nothing. Not a wrong answer — no answer, which is worse, because a wrong answer at least tells you to look again.
Stop — the test never happened
The ethanol caught fire.
Ethanol boils at 78 °C and its vapour ignites readily. Held over a Bunsen it goes up, and the fire is at head height in a room full of people. This is the one mistake on this bench that is not about the quality of your data.
Can you conclude anything? No. Stand the tube in hot water and let the water carry the heat — the water bath is not fussiness, it is the reason this practical is allowed in schools at all.
A result you can use
Blue-black in the exposed half. Orange-brown under the foil.
Both halves are on one leaf, so they shared the same water, temperature, carbon dioxide and age. Light is the only difference between them, and starch appeared only where light fell.
Can you conclude anything? Yes: light is needed for photosynthesis in this plant. The covered half is the control, and it is the only reason that sentence is allowed to contain the word "needed".
Result partly readable
The leaf crumbled as you spread it.
Ethanol leaves the leaf brittle and shrivelled. It tore along the fold and part of the covered half is now in three pieces, so the boundary you were trying to look at is gone.
Can you conclude anything? Partly. The chemistry worked and the fragments do show a difference, but the pattern is what this experiment is about, and you have damaged the pattern.
The method, and why each step is in it
Five steps. None of them optional.
Destarch the plant
Two days in the dark with no light to photosynthesise by. The plant respires away the starch it was holding, so anything found afterwards was made during your experiment. This is the control, and without it no result means anything.
Boil the leaf in water
Kills the cells so nothing changes while you work, and breaks down the membranes so that iodine can get into the cells later. It also softens the waxy surface.
Boil in ethanol, in a water bath
Ethanol dissolves chlorophyll, so the green drains out and a colour change becomes visible. Ethanol boils at 78 °C and its vapour catches fire easily, so it is heated by standing the tube in hot water — never over a flame.
Dip in hot water
Ethanol leaves the leaf pale, brittle and shrivelled. A few seconds in hot water softens it enough to spread flat without it crumbling.
Spread out and add iodine
On a white tile, so the colour is read against a neutral background. Blue-black where starch is present, orange-brown where it is not.
Key fact
Iodine solution turns blue-black in the presence of starch and stays orange-brown without it. A leaf must be destarched first, then killed, decolourised and softened, or the result cannot be read or trusted.
Think again
“The leaf goes black because the iodine reacts with the chlorophyll.”
The chlorophyll is not there. It was dissolved out into the ethanol in step 3, which is the step that turns the ethanol green and the leaf pale, and the iodine does not go on until step 5 — by which time the green pigment is in a beaker on the other side of the bench. Two more observations finish the argument. A slice of potato has no chlorophyll and never has had, and iodine turns it blue-black instantly; so does a piece of bread. And the white parts of a variegated leaf, which have no chlorophyll to react with, stay orange-brown when the green parts go black — the opposite of what this idea predicts. Iodine is a test for starch, nothing else, and that is why it is used identically in Food tests on food that was never green.
“Just pick a leaf and test it — the destarching is a waste of two days.”
Then a positive result proves nothing, because you cannot tell whether the starch was made today under the conditions you were testing or last Tuesday in ordinary daylight. Starch is the storage form: a leaf accumulates it over days and holds onto it. Destarching — the whole plant in a dark cupboard for two days, respiring away its store with no light to replace it — is not preparation, it is the control. It is what lets you say the starch you found was made during the experiment. This is exactly the fault you met as no comparison group in Substance misuse and decisions, wearing different clothes: a measurement with nothing to compare it against is not a result.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Why that step
What is the purpose of boiling the leaf in ethanol?
Rung 2 · The one that catches people
A leaf is picked straight from a plant on a windowsill, tested properly, and goes blue-black. What has been shown?
Rung 3 · Write the method
Write the method for testing a leaf for starch as instructions someone else could follow, giving a reason for each step. Include the safety point.
Rung 4 · Take it somewhere new
Design an investigation using this test to show that carbon dioxide is needed for photosynthesis. Say what your two set-ups are, what you keep the same, and what result would support your hypothesis.
Key note
Destarch the plant in the dark for two days. Boil the leaf in water to kill it and make it permeable, boil it in ethanol in a water bath to remove the chlorophyll, dip it in hot water to soften it, spread it on a white tile and add iodine. Blue-black means starch; orange-brown means none. Never heat ethanol over a naked flame.
Going further
Photosynthesis makes glucose, so why does the test look for starch? Because glucose is soluble, and a cell that filled itself with dissolved sugar would pull water in after it and swell hard against its wall — a leaf storing its day's production as glucose would be a bag of syrup under pressure. Starch is hundreds of glucose molecules joined into one insoluble molecule: it sits in the chloroplast as a grain, takes up little room, pulls in no water at all, and can be broken back down to glucose when the plant needs it at night. That is also why it stays put long enough to be found the next morning, which is the only reason this practical works. The same argument explains why animals store glucose as glycogen rather than as sugar in the blood.
Before this lesson
Connects to
- Leaves built for the job
- Substance misuse and decisions
Where the same fault — a measurement with nothing to compare it against — is met in evidence about people rather than in a leaf.
At GCSE this becomes
- Required practical work on photosynthesis, limiting factors, and writing a method that another person could follow exactly.
Where to next
Ask Mr Badmus AI
Want to check whether your result actually shows what you think?
The bench is a simulation of the practical, not a substitute for it, and it is not a risk assessment. In a real laboratory: ethanol is highly flammable and is heated only in a water bath with no naked flame in the room; iodine solution stains and irritates skin and eyes; boiling water and hot apparatus need eye protection and a teacher present. Follow your school's own risk assessment.
Lesson content © MrBadmusAI.