The Red Cabbage pH Indicator

Chemistry you can see

Some chemistry is invisible — but not this. With a single red cabbage you can make a liquid that changes through a rainbow of colours depending on what you add to it. Drop in vinegar and it flushes pink; add baking soda and it turns blue-green. That colourful liquid is a pH indicator, and making one is a brilliant way to understand acids and bases.

What are acids and bases?

Almost every liquid around you is either an acid, a base, or neutral.

• Acids taste sour (think lemon, vinegar and stomach acid). They contain lots of tiny particles called hydrogen ions.
• Bases (also called alkalis when they dissolve in water) feel slippery and include baking soda, soap and ammonia. They have very few hydrogen ions.
• Neutral substances, like pure water, sit in the middle.

Chemists measure how acidic or basic something is using the pH scale, which runs from 0 to 14:

pH	Type	Examples
0–6	Acidic	Lemon juice, vinegar, fizzy drinks
7	Neutral	Pure water
8–14	Basic	Baking soda, soap, bleach

The lower the number, the stronger the acid; the higher the number, the stronger the base. You can read more in Acids and Bases.

Why red cabbage works

Red cabbage is purple because of pigments called anthocyanins. These molecules are special: their shape — and therefore the colour of light they reflect — changes depending on the acidity around them. In acid they look red or pink; in neutral conditions they stay purple; in a base they shift to blue, green and finally yellow. That makes red cabbage a natural, edible indicator that maps the whole pH scale onto colours your eyes can read.

Make the indicator

You will need: half a red cabbage, a pot, water, a strainer, a clear jar, and an adult to help.

1. Chop the cabbage into small pieces and put them in a pot.
2. Cover with water and ask an adult to boil it for about 10 minutes, until the water turns deep purple.
3. Let it cool, then strain out the cabbage. The dark purple liquid that remains is your indicator. Keep it in a jar.

Safety: an adult must handle the hot pot and water. The indicator liquid is safe to touch, but treat it as a science material — do not drink your test mixtures.

Run the experiment

Line up several clear cups. Pour a little cabbage indicator into each, then add a different test liquid to each cup:

• Lemon juice or vinegar (acids)
• Baking soda dissolved in water (a base)
• Plain water (neutral)
• Milk (slightly acidic)
• Soapy water (basic)

Watch each cup change colour. Record what you see, like a real chemist:

Test liquid	Colour	Acid, neutral or base?
Lemon juice	Bright pink/red	Acid
Vinegar	Pink/red	Acid
Water	Purple	Neutral
Baking soda	Blue/green	Base
Soapy water	Green/yellow	Strong base

Important safety rule: test only mild kitchen liquids. Never test bleach, drain cleaner or other strong household chemicals, and never mix cleaning products together — some combinations release toxic gas.

Why does the colour change?

It all comes down to hydrogen ions — the same particles that define acids and bases. When you add an acid, the cup fills with hydrogen ions, and the anthocyanin molecules change shape to reflect red light. When you add a base, hydrogen ions are scarce, and the molecules shift to reflect green and yellow. Neutral liquids leave them in their middle, purple form. So the colour you see is a direct readout of how many hydrogen ions are in the cup. You are literally watching the pH scale.

A neutralisation surprise

Try this finishing trick. Take a cup that turned pink with vinegar (an acid) and slowly stir in a little baking-soda water (a base). The colour shifts back toward purple, and you may see fizzing. The acid and base are neutralising each other, cancelling out toward the middle of the scale and producing carbon dioxide gas. This is the same kind of reaction that settles an acid stomach — and a clear example of a chemical change. Explore that idea further in Chemical Reactions and Signs of Change.

Make it a fair test

To turn this into proper science, control your variables. Use the same amount of indicator in every cup, add the same amount of each test liquid, and test at the same temperature. Then the only thing changing is the liquid itself — so any colour difference must come from its acidity. That is what makes results trustworthy. Learn the technique in What Is a Fair Test? Variables Explained.

Why this matters

Indicators are not just a fun trick. Scientists, gardeners, pool owners and water companies all measure pH every day. Soil pH decides which plants will grow; blood pH must stay in a narrow safe range; and factories must check that wastewater is neither too acidic nor too basic before releasing it. Your red-cabbage cup is a simple version of tools used in laboratories worldwide — proof that real chemistry can begin in a kitchen.