How do you find starch in a substance?

In one line

The rule

Starch gives a blue-black colour when treated with iodine solution.

This is called the iodine test. It is a simple chemical test for detecting starch, a complex carbohydrate made from many glucose units.

Let's understand it simply

Imagine starch as a hidden message in food. Iodine acts like an ink that makes this message visible. If starch is present, the mixture turns blue-black.

If starch is absent, the iodine usually stays yellow-brown. This colour change is not caused by all carbohydrates. For example, glucose does not give the blue-black colour in this test.

Starch is found in foods such as rice, wheat, potato, bread, and corn. The test helps us check these foods quickly.

How it works

1. Take the sample. Put a small amount of the substance on a white tile or in a clean test tube.

1. Make a paste if needed. Add a little clean water and mix solid food, such as potato or bread.

1. Add iodine solution. Use a dropper to add two or three drops.

1. Watch the colour. Look at the sample immediately after adding iodine.

1. Read the result. A blue-black colour means starch is present. A yellow-brown colour usually means starch was not detected.

1. Compare carefully. Use a known starch sample, such as rice water, and plain water for comparison. This makes the result easier to judge.

The story behind it

Scientists developed chemical tests to identify substances in food and living tissues. These tests helped biology move from simply observing food to studying its chemical composition.

The iodine test became a useful school laboratory test because it is quick, needs only a small sample, and gives a clear colour change. It connects with the study of biomolecules (chemical substances made by living organisms), including carbohydrates.

Where you see it in real life

• Potato → blue-black colour → potato stores energy mainly as starch.
• Cooked rice → blue-black colour → rice contains a large amount of starch.
• Bread → blue-black colour → flour contains starch.
• Sugar solution → no blue-black colour → ordinary sugar is not starch.
• A leaf → blue-black patches after testing → those patches show where starch formed during photosynthesis.
• Food testing in a laboratory → colour change → scientists use chemical tests to identify food nutrients.

Why it matters

• It helps you identify starch in common foods.
• It is used in school experiments on photosynthesis.
• It helps compare different food samples.
• It introduces the study of carbohydrates and other biomolecules.
• It teaches an important science habit: use evidence, not guessing.

Common mistakes

• Using too much iodine: A very dark brown mixture can make the result hard to see. Use only a few drops.
• Calling every carbohydrate starch: Glucose and simple sugar do not give the blue-black starch test.
• Using a dirty container: Old food or chemicals may change the colour. Use clean equipment.
• Touching or tasting iodine: Iodine solution is a laboratory chemical. Do not taste it, and wash spills quickly.

Remember it

Iodine + starch = blue-black.