Grade 11 · 🧪 Chemistry · Acids & Bases · CHE-G11-AB1.1
Make your own pH indicator from red cabbage
Boil red cabbage to make a colour-changing indicator, then test lemon juice, vinegar, baking soda, soap and ash water.
Why this matters
Injera tastes sour because fermentation lowers the batter's pH from about 7 to 4. Farmers in the Oromia highlands check soil pH because teff grows best between 5.5 and 7.5. Doctors monitor blood pH, which must stay between 7.35 and 7.45.
Watch the demo
Video by Royal Institution
The science
Acids and bases
An acid donates a proton (H⁺). A base accepts a proton. Lemon juice and vinegar are acids; baking soda, soap and ash water are bases.
The pH scale
pH = −log₁₀[H⁺]. Pure water has pH 7 (neutral). pH < 7 is acidic, pH > 7 is basic. Each pH unit is a factor of 10 in [H⁺].
Indicators
An indicator is a substance whose colour depends on pH. Red cabbage contains anthocyanin, a pigment that changes shape — and colour — as [H⁺] changes.
You'll need
- Half a red cabbage↳ Beetroot works too, with weaker colours
- Hot water and a pot
- 3–5 clear cups
- Test substances: lemon juice, vinegar, baking soda, soap water, ash water↳ Ash water: stir a spoon of wood ash into water and let it settle
Safety first
- Ask an adult to help with boiling water.
- Ash water is a strong base and can irritate skin and eyes. Handle it with a spoon, not your fingers, and wash hands afterwards.
- Never taste the test solutions.
Step by step
0/4 doneWhat you should see
Lemon juice and vinegar turn the indicator pink or red. Plain indicator stays purple. Baking soda and soap water turn it blue or green. Ash water turns it green or yellow.
Why it happens
Anthocyanin changes colour with pH: pink/red means strongly acidic (pH 1–3), purple is near neutral (about 7), blue/green is mildly basic (9–11), and yellow is strongly basic (12+). Universal indicator paper works the same way, using a mix of pigments.
Didn't work? Try this
The cabbage water is pale.
Use less water or boil a little longer. More concentrated indicator shows clearer colours.
All the cups look the same purple.
Add more of each test substance, and view the cups against a white sheet of paper.
Still stuck? Ask the lab assistant
Check your understanding
1. A solution has pH = 4. How does its [H⁺] compare to one with pH = 6?
2. The indicator turns green when you add soap water. Soap water is…
Your lab notebook
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