What it shows
A drop of diluted tincture of iodine turns blue black on anything that contains starch, such as potato, rice and bread, and stays brown on foods without starch, such as cheese or oil. It is a quick chemical test that sorts foods and can expose starch added to milk or spices.
The science
Starch is made of long coiled chains of glucose. Iodine molecules slip inside the coils and the trapped iodine absorbs light differently, giving the deep blue black colour. Foods that store sugar instead of starch, such as ripe fruit, or that are mostly protein or fat, have no coils to trap the iodine, so the drop keeps its original brown colour. Ripening fruit turns starch into sugar, which is why a green banana tests dark and a ripe one tests lightly.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Tincture of iodine from a first-aid box, 10 drops in 2 spoons of water (adult dilutes)
- A dropper
- Small pieces of potato, rice, bread, banana, apple, paneer, boiled egg white and a spoon of oil
- A white plate
- Paper and pencil for the results table
Safety
For this project: An adult must dilute and handle the iodine; it stains skin and clothes and must never be swallowed or put near eyes. Throw away all tested food.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Ask an adult to mix the iodine drops into water so the solution is the colour of tea.
Arrange the food pieces around the white plate and write their names on paper beside them.
Put one drop of the solution on each food and wait thirty seconds.
Record the colour for each food: blue black means starch is present, brown means it is absent.
Test a green banana and a ripe banana side by side and compare the darkness.
Put a drop of iodine on a spoon of milk and on a spoon of milk with a pinch of cornflour stirred in, to see how adulteration is detected.
Make a table with three columns: food, colour, starch present or absent.
Rinse the plate thoroughly and wash hands when finished.
What you should see
Potato, rice, bread and green banana turn blue black; apple, paneer, egg white, oil and plain milk stay brown; the milk with cornflour turns blue, showing the test can catch added starch.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What colour shows starch?
Blue black.
2Why does ripe banana test lighter than green banana?
Ripening changes starch into sugar, which does not react with iodine.
3Why does iodine turn dark with starch?
Iodine fits inside the coiled starch chains and the trapped iodine looks blue black.
4How can this test catch adulterated milk?
Pure milk has no starch, so a blue colour means starch or flour was added.
Take it further
Chew a piece of bread for a minute, then test it; saliva begins turning starch into sugar and the colour is weaker.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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