Photosynthesis Experiments and the Starch Test (ICSE Class 10 Biology)
Updated 15 October 2026 · Fact-checked
To show a factor is needed for photosynthesis, destarch a plant by keeping it in darkness for 24 to 48 hours, deprive one leaf or part of that factor, expose the plant to light, then test for starch: boil in water, boil in alcohol, wash, add iodine. Parts that photosynthesised turn blue-black; deprived parts stay brown.
Understand Photosynthesis Experiments and the Starch Test
The logic of every experiment. Starch is the visible product of photosynthesis in a leaf. If a leaf part makes starch, it photosynthesised; if it does not, something essential was missing. Each experiment therefore removes one factor from one part and compares it with a part that has everything (the control).
Destarching. Keep the potted plant in complete darkness for 24 to 48 hours before the experiment. With no light, the leaves cannot photosynthesise, and the starch already present is converted to sugar and moved away or used. This ensures that any starch found later was made during the experiment.
The starch test (steps and reasons).
| Step | Reason |
|---|---|
| 1. Boil the leaf in water for a few minutes | kills the cells, stops enzyme action, softens the leaf and makes cells permeable to iodine |
| 2. Boil in alcohol (methylated spirit) in a test tube placed in a hot water bath | removes chlorophyll, so colour changes are easy to see; water bath because alcohol is inflammable |
| 3. Dip in warm water | alcohol makes the leaf brittle; warm water softens it |
| 4. Spread on a tile and add dilute iodine solution | starch turns blue-black; parts without starch stay brown (colour of iodine) |
Experiment 1: light is necessary. Cover part of a leaf of a destarched plant on both sides with a strip of black paper (or a stencil), keep the plant in sunlight for some hours and test the leaf for starch. The covered part stays brown (no starch); the exposed part turns blue-black.
Experiment 2: carbon dioxide is necessary (Moll's half-leaf experiment). Insert half of a leaf of a destarched plant into a wide-mouthed bottle containing potassium hydroxide (KOH) solution, through a split cork, leaving the other half outside. KOH absorbs CO₂ from the air in the bottle. Keep the plant in sunlight and test the leaf. The part inside the bottle stays brown; the part outside turns blue-black. (The part held in the cork also gets no CO₂.)
Experiment 3: chlorophyll is necessary. Use a variegated leaf (green and white or yellow, such as croton or coleus). Draw its pattern, destarch the plant, keep it in sunlight and test. Only the green parts turn blue-black; the white parts stay brown.
Experiment 4: oxygen is released. Place Hydrilla (a water plant) in a beaker of pond water with a little sodium bicarbonate (to supply CO₂). Cover it with an inverted funnel whose stem is under water, and invert a test tube full of water over the funnel stem. Keep the set-up in sunlight. Bubbles rise and gas collects at the top of the test tube, pushing the water down. A glowing splinter bursts into flame in the gas: it is oxygen.
Flow chart. Destarch (dark 24-48 h) → set up experiment and control → sunlight for some hours → pluck leaf → boil in water → boil in alcohol (water bath) → warm water → iodine → blue-black = starch present; brown = absent.
Draw-and-label checklist: Hydrilla experiment (as expected in 2025). Beaker of water with the water level above the funnel stem; Hydrilla shoots under an inverted funnel; an inverted test tube over the funnel stem; bubbles of oxygen rising; gas collected at the top of the test tube; sunlight rays shown. Labels: beaker, water, Hydrilla, funnel, test tube, oxygen bubbles, oxygen gas, sunlight.
Draw-and-label checklist: Moll's experiment. Potted plant; one leaf passing through a split cork into a bottle; KOH solution in the bottle; leaf half inside and half outside; label KOH (absorbs CO₂), split cork, inside part (no CO₂), outside part.
Photosynthesis Experiments and the Starch Test: key facts
- Starch test steps
Boil in water kill cells, soften Boil in alcohol on water bath remove chlorophyll Warm water soften brittle leaf Iodine blue-black if starch - Order and reason for each step are both examined.
- Factor, method, result
Light black paper strip, covered part brown CO₂ KOH bottle, inside part brown Chlorophyll variegated leaf, white part brown O₂ release Hydrilla, gas relights glowing splinter - Brown means no starch; blue-black means starch.
- Chemicals and their roles
KOH absorbs CO₂ Sodium bicarbonate supplies CO₂ in water Alcohol dissolves chlorophyll Iodine tests for starch - Do not confuse KOH (removes) with bicarbonate (adds).
How to answer Photosynthesis Experiments and the Starch Test questions
ICSE gives an experiment diagram or a paragraph with blanks and asks for the aim, the purpose of a step or chemical, the observation, the control and the conclusion, or asks you to draw the Hydrilla set-up.
- 1Identify which factor is removed: light (paper), CO₂ (KOH), chlorophyll (variegated), or which product is shown (O₂).
- 2State why the plant was destarched.
- 3Give the colour of each part after iodine: deprived part brown, normal part blue-black. Read which part is asked first.
- 4Explain each starch-test step with its reason.
- 5In a diagram, keep the water level above the funnel stem, show bubbles, the test tube and sunlight.
Photosynthesis Experiments and the Starch Test in real life
The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.
Patterns on a leaf
Suppose a science club wants to 'print' its logo on a living leaf. They cut the logo out of black card, clip the card to both sides of a leaf on a plant that was kept in a dark cupboard for two days, and leave it in sunlight for a day.
What should they do next to see the logo, and how will it appear?
Try it first, then show the solutionSolution
- Pluck the leaf, boil it in water, then in alcohol on a water bath to remove chlorophyll, and soften it in warm water.
- Spread it out and pour iodine solution over it.
- Where light passed through the cut-out, the leaf made starch and turns blue-black.
- Where the card blocked light, no starch formed and the leaf stays brown.
Answer: After the starch test the logo appears blue-black on a brown background, showing light is needed for photosynthesis.
Memory hook: Light writes in starch; iodine reads it.
Tricks to remember Photosynthesis Experiments and the Starch Test
Short memory aids. Say each one aloud once, then use it on a question the same day.
W-A-W-I
Water, Alcohol, Warm water, Iodine: 'We Always Wash It'.
Use it when: Order of the starch test.
KOH Kills CO₂
K-O-H keeps out CO₂; bicarbonate brings it back.
Use it when: Roles of the two chemicals.
Brown = Barred
The barred (deprived) part stays brown; the free part goes blue-black.
Use it when: Not swapping colours.
Photosynthesis Experiments and the Starch Test: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Fill the blanks: CO₂ experiment · 3 marks · Short answerBoard level
To test whether leaves need carbon dioxide to make starch, a student first leaves a potted plant in a dark cupboard for two days to ______ it. She then seals half of one leaf inside a bottle containing ______ solution, which removes ______ from the air around it. After some hours in sunlight the leaf is tested with iodine: the half inside the bottle turns ______ and the half outside turns ______. (The 2024 paper set a fill-in on this experiment.)
Try it first, then show the solutionSolution
- Destarch; potassium hydroxide; carbon dioxide (1½ marks).
- Inside: brown (no CO₂, no starch); outside: blue-black (1½ marks).
Answer: Destarch; KOH; CO₂; brown; blue-black.
Example 2: Reasons for starch-test steps · 3 marks · Short answerHarder board level
Give the reason for each step: (a) boiling the leaf in water, (b) boiling it in alcohol over a water bath, (c) dipping it in warm water.
Try it first, then show the solutionSolution
- (a) Kills the cells and softens the leaf so iodine can enter (1 mark).
- (b) Removes chlorophyll so the colour change is visible; a water bath is used because alcohol catches fire easily (1 mark).
- (c) Softens the leaf, which alcohol has made brittle (1 mark).
Answer: (a) Kill and soften. (b) Decolourise safely. (c) Soften the brittle leaf.
Example 3: Variegated leaf · 2 marks · Short answerBoard level
A destarched croton plant with green-and-yellow leaves is kept in sunlight and one leaf is tested with iodine. State the observation and conclusion.
Try it first, then show the solutionSolution
- Green parts turn blue-black; yellow parts stay brown (1 mark).
- Only parts with chlorophyll made starch, so chlorophyll is necessary for photosynthesis (1 mark).
Answer: Green parts blue-black, yellow parts brown; chlorophyll is necessary.
Example 4: Hydrilla set-up · 3 marks · Short answerHarder board level
Using a water plant, a glass funnel and a test tube, draw a labelled diagram of an experiment that shows a gas is given out during photosynthesis. Name the gas and the test for it. (The 2025 paper asked for a diagram of this set-up.)
Try it first, then show the solutionSolution
- Beaker of water with Hydrilla under an inverted funnel, water level above the funnel stem (1 mark).
- Inverted test tube over the funnel stem with gas collected at the top and bubbles rising (1 mark).
- Sunlight shown; labels: beaker, water, Hydrilla, funnel, test tube, oxygen (1 mark).
Answer: Correct diagram with all parts and labels; the gas is oxygen, which relights a glowing splinter.
Example 5: Missing destarching · 3 marks · Case-basedHOTS / competency
Suppose a student does the black-paper experiment without destarching the plant, and finds that the covered part also turns blue-black. (a) Explain her result. (b) Does it prove light is not needed? (c) How should she repeat the experiment?
Try it first, then show the solutionSolution
- (a) Starch made before the experiment was still in the leaf, including the covered part (1 mark).
- (b) No: the starch was not made during the experiment, so the result says nothing about light (1 mark).
- (c) Keep the plant in darkness for 24 to 48 hours first, check one leaf to confirm it has no starch, then repeat (1 mark).
Answer: (a) Old starch. (b) No. (c) Destarch first and confirm with a test.
Example 6: Improving the Hydrilla experiment · 2 marks · Short answerHOTS / competency
Suppose few bubbles form in a Hydrilla set-up kept in a shady room. Suggest two changes that would increase the bubbling and explain each.
Try it first, then show the solutionSolution
- Move it into bright sunlight: more light energy, faster photosynthesis (1 mark).
- Add a pinch of sodium bicarbonate: more CO₂ dissolved in the water for photosynthesis (1 mark).
Answer: Brighter light and added sodium bicarbonate (or warm the water slightly towards the optimum).
Where students lose marks in Photosynthesis Experiments and the Starch Test
The slips examiners see most often, and what to write instead.
Mistake: Swapping brown and blue-black
CISCE examiners' comments (2024): in the CO₂ experiment fill-in, candidates wrote blue-black for the part inside the flask and brown for the part outside, reversing the answer.
Wrong working
Inside the flask: blue-black; outside: brown.
Right working
Inside the flask: brown; outside: blue-black.
Do this: Ask: did this part get the factor? No factor, no starch, brown.
Usually costs: 1-2 marks.
Mistake: Poor Hydrilla diagram
CISCE examiners' comments (2025): candidates merged the funnel and test tube, did not show oxygen collecting in the test tube, drew the water level below the funnel stem, omitted sunlight, or drew unrelated diagrams of stomata or chloroplasts.
Do this: Practise the set-up with the water level above the stem and sunlight drawn.
Usually costs: 1-3 marks.
Mistake: No reason for destarching
Writing 'to destarch' without 'so that any starch found was made during the experiment' loses the explanation mark.
Do this: Always add the purpose.
Usually costs: 1 mark.
Exam tips
- Write colours for both parts and say which is which.
- Give the purpose of each chemical: KOH, bicarbonate, alcohol, iodine.
- Name the control in every experiment answer.
- Draw sunlight in photosynthesis set-ups.
Practice questions on Photosynthesis Experiments and the Starch Test
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · MCQBoard level
Before an experiment on photosynthesis, a plant is kept in darkness to:
(a) increase chlorophyll (b) remove starch from the leaves (c) open stomata (d) absorb CO₂Hint
Destarching.
Answer
(b) remove starch from the leaves.
Q2 · 1 mark · Very short answerBoard level
Name the chemical used to absorb carbon dioxide in the half-leaf experiment.
Hint
An alkali.
Answer
Potassium hydroxide (KOH).
Q3 · 2 marks · Short answerHarder board level
Why is the leaf covered with black paper on both surfaces, not only on the upper surface?
Hint
Light from below.
Answer
So that no light reaches the covered part from either side; otherwise light entering from below could let that part photosynthesise and spoil the comparison.
Q4 · 1 mark · Very short answerHarder board level
In the Hydrilla experiment, how will you prove that the gas collected is oxygen?
Hint
Splinter test.
Answer
A glowing splinter put into the gas bursts into flame.
Q5 · 2 marks · Short answerHarder board level
Which part of the leaf acts as the control in the CO₂ experiment? Why is a control needed?
Hint
The part outside.
Answer
The part outside the bottle, which gets CO₂. The control shows the leaf can make starch under the same conditions, so the only difference (no CO₂) explains the result inside.
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A leaf is boiled in alcohol over a water bath, not directly over a flame.
Reason (R): Alcohol is inflammable.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).Hint
Safety.
Answer
(c).
Q7 · 2 marks · Short answerHarder board level
Why is sodium bicarbonate added to the water in the Hydrilla experiment?
Hint
Raw material.
Answer
It releases carbon dioxide into the water, so the plant has enough CO₂ for photosynthesis and gives off more oxygen.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a destarched plant has one leaf half-covered with black paper and a second leaf half-inserted into a KOH bottle, and the whole plant is kept in sunlight for six hours.
(a) Which parts will turn blue-black? (1)
(b) Name the factor tested in each leaf. (1)
(c) If the plant had been kept in the dark for the six hours instead, what result would you expect? (1)Hint
Every factor must be present.
Answer
(a) The uncovered half of the first leaf and the half outside the bottle of the second leaf. (b) First leaf: light; second leaf: carbon dioxide. (c) No part would turn blue-black, since no photosynthesis occurs in the dark.
Test your recall
Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.
1.What colour does iodine give with starch?Show answer
Blue-black.
2.For how long is a plant usually destarched?Show answer
24 to 48 hours in darkness.
3.Which leaf is used to show chlorophyll is necessary?Show answer
A variegated leaf (croton, coleus).
4.Which plant is used to show oxygen release?Show answer
Hydrilla.
5.Why is the leaf boiled in alcohol?Show answer
To remove chlorophyll.
Photosynthesis Experiments and the Starch Test: frequently asked questions
Why do we test for starch and not glucose?
Glucose made in the leaf is quickly converted to starch for storage, and starch gives a clear blue-black colour with iodine.
Can a leaf be tested without boiling in water?
Boiling in water kills the cells and softens them so iodine can reach the starch; without it the test is unreliable.
Why use Hydrilla for the oxygen experiment?
It is a water plant, so the oxygen it releases forms visible bubbles that can be collected over water.
Is the gas collected pure oxygen?
Mostly oxygen, with a little dissolved air, but enough oxygen to relight a glowing splinter.