Starch test
- ICSE Class 10, Unit 2 Plant Physiology
The starch test checks whether a leaf has made food by photosynthesis: a leaf with starch turns blue-black when iodine solution is added.
Each step has a reason. Learn both.
- Destarch: keep the plant in the dark for 48 hours, so the leaves use up the starch they had stored.
- Boil the leaf in water, to kill its cells and make it soft.
- Boil it in alcohol over a water bath, to remove the green chlorophyll. Then colour changes are easy to see.
- Rinse it in warm water and add iodine solution. Blue-black means starch. Brown, the colour of the iodine, means none.
A small example
Destarch a plant. Put part of one leaf inside a flask with potassium hydroxide, which soaks up carbon dioxide. Leave the plant in sunlight, then test the leaf. The part in the flask stays brown. The part outside turns blue-black. So photosynthesis needs carbon dioxide.
Where marks go
The 2024 ICSE Biology paper asked for these two colours in a fill-in-the-blanks question. CISCE's report on the 2024 ICSE Biology paper says some answers swapped them. They gave blue-black for the part inside the flask, and brown for the part outside.
When a question asks why, the marking scheme gives marks for the reason. Why is the plant kept in the dark for 48 hours? Why is the leaf boiled in alcohol? Give the reason, not the order of steps. The ICSE 2027 specimen paper asks for the aim and conclusion of the same experiment done under a bell jar.
Sources
Facts last checked against these sources on 30 September 2026.
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