What's the iodine actually doing?

• Betadine contains iodine — and iodine is one of nature's best detectives. When iodine meets starch, something dramatic happens: the two lock together and the colour instantly changes from brown to dark blue-black. 

• When there is no starch, iodine stays brown and moves on. It's not reacting with the sugar, the water, the colour, or the flavour of the food — only the starch. 

• That makes it what scientists call an indicator: a substance that changes colour only when it finds one specific thing. The dark blue-black is your evidence. Brown means innocent.

Why do plants make starch?

• Starch is how plants store energy. Plants make a simple sugar called glucose during photosynthesis — but storing lots of sugar would make them sticky and draw insects and bacteria.

• Instead, plants join thousands of glucose molecules together in a long chain that coils up tightly, like a spring. That coiled-up chain is starch.

•  It has no taste, no colour, and no smell — completely invisible to every one of your senses. It's the plant's way of packing a huge amount of energy into a small, stable, hidden package. 

• The foods in your experiment that test positive are the ones where the plant stored a lot of starch — usually in seeds, roots, and underground parts where the plant keeps its energy reserves.

Why does iodine change colour inside starch?

• When iodine meets a starch coil, it doesn't sit on the surface — it slips inside the spiral, like threading a string through a hollow tube. Once iodine is trapped inside that coil, it absorbs light differently. 

• Instead of looking brown, it looks dark blue-black. The colour change is instant, reliable, and dramatic. When there's no starch coil to slip into — like in apple flesh, butter, or cheese — the iodine has nowhere to go. It stays brown and nothing changes. This test is so reliable that scientists still use it in food labs, hospitals, and agricultural testing around the world.

Real-world connection

• Indicators like iodine are used everywhere. Hospital tests use indicator chemicals to check blood and urine for substances that shouldn't be there. Swimming pool owners use them to check if the water is safe. 

• Food manufacturers use them to check starch levels in crops before harvest. What they all have in common is the same idea you used today: a chemical that changes colour to reveal something invisible — giving you a clear answer without any guessing.

Try next

• See another chemical test used to reveal something hidden inside a living thing → [The Escape Artist]

• Find out where the glucose that makes starch originally comes from → [A Plant's Hidden Pipes]