Why does red cabbage change colour?

At first, your red cabbage juice looked like nothing more than purple water.

But hidden inside the cabbage are natural pigments called anthocyanins. These pigments are extremely sensitive to the chemicals dissolved in different liquids.

When you added your homemade red cabbage pH indicator to each mystery liquid, the pigments changed shape. That tiny change affected the colour of the light they reflected, causing the solution to turn pink, purple, blue or green.

This is why red cabbage is one of the best natural pH indicators you can make at home.

What do acids and bases do?

Every liquid contains different amounts of tiny particles called hydrogen ions.

Acidic liquids contain more hydrogen ions, causing the cabbage indicator to turn shades of pink or red.

Neutral liquids, like pure water, usually stay purple because there is a balance of hydrogen and hydroxide ions.

Basic (alkaline) liquids, such as baking soda solution or some soaps, contain fewer hydrogen ions and often turn the indicator blue or green.

Scientists use this colour change to estimate a liquid's pH, which tells them whether it is acidic, neutral or alkaline.

Why did your results change?

During your investigation, you tested different household liquids while keeping your testing method the same.

Each liquid contained different chemicals, so each one reacted differently with the cabbage indicator.

By comparing the colours, you weren't just watching a colour change—you were collecting evidence.

This is exactly how scientists investigate unknown substances. They test one sample at a time, compare the results and use evidence to reach a conclusion.

Curiosity Spark

Red cabbage isn't the only plant that can reveal hidden chemistry.

Some flowers, berries and even autumn leaves contain the same family of pigments called anthocyanins.

Scientists also use special indicators to monitor swimming pools, drinking water, aquariums, soil, laboratories and even medical tests.

If an ordinary cabbage can reveal so much about a liquid...

What other plants do you think could become natural science tools?

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