Red cabbage juice is a homemade acid–base indicator. The purple pigments in its leaves change color when you add an acidic liquid such as lemon juice or a basic solution such as diluted baking soda. You can demonstrate this with clear cups, cabbage, water, and a few ordinary kitchen ingredients.
What colors will red cabbage indicator show?
This photograph shows seven solutions containing the same red cabbage indicator, arranged from more acidic on the left to more basic on the right. Pink and red suggest acidic conditions. Purple to blue appears nearer neutral. Green or yellow usually indicates increasingly basic conditions. The exact color depends on concentration, lighting and the ingredients tested, so treat the result as a rough classification rather than a precise pH number.
Gather simple materials
You need two or three red cabbage leaves, approximately one cup of room-temperature water, a sealable food-storage bag, four or five transparent cups, a strainer or sieve, and a spoon or dropper. For test samples, use plain water as a control, a little lemon juice or vinegar, and a small amount of baking soda dissolved in water. Put a towel under the cups; the pigment can stain.
This version follows the American Chemical Society’s no-boiling-water classroom approach, which is well suited to an adult-supervised kitchen-table experiment. Use clean containers for each sample but do not use the mixtures for food afterward.
Make the indicator without a stove
Step 1: Tear the cabbage leaves into small pieces and place them in the bag. Add about one cup of water, remove excess air and seal. Step 2: Gently squeeze and knead the bag for a couple of minutes. The water should turn bluish purple as pigment enters it. Step 3: Strain the colored liquid into a clean cup and discard the cabbage solids. Step 4: Divide small equal portions of the purple liquid among separate clear cups.
The colored liquid is your indicator. Before testing, place a plain-water cup beside the others so you can compare changes against the starting color. Strongly colored juices, opaque cleaners and unknown household chemicals are poor choices: their color or hazards can obscure what you are trying to learn.
Test one ingredient at a time
Add several drops of lemon juice to the first sample and stir. It should trend pink or red. Add a little diluted baking soda solution to a second sample. It may become blue or green. Leave a third sample with only water. Compare the cups against a white background and record their differences before adding more of anything.
If a sample does not change much, the test substance might be weak, the cabbage extract too dilute, or the background color difficult to judge. Try fresh indicator and a small measured amount rather than adding lots of different chemicals to the same cup. Never combine bleach, ammonia, drain cleaner or other strong household products in this experiment.
Why does the pigment change color?
The pigment family is called anthocyanins. Molecules in this family can adopt different structures depending on the surrounding acidity, changing which wavelengths of light they absorb. Our eyes see those chemical differences as pink, purple, blue or green. The test reveals a chemical property of the liquid, not merely a dye spreading through water.
The same principle helps explain why purple cabbage sometimes changes shade in recipes containing vinegar or other ingredients. That kitchen behavior is covered separately in why red cabbage changes color while cooking. This experiment focuses on the scientific measurement concept.
Make it a genuine science investigation
Have everyone predict each sample’s color before testing. Label the cups, keep their volumes similar and use clean spoons for each liquid. Test a series of increasingly diluted lemon-juice solutions to see whether the color becomes less intense as the acidity changes. Repeat with fresh cabbage extract and compare the results. Record the results as acid, near-neutral or base, avoiding unsupported exact pH claims.
Safety and cleanup
An adult should supervise; children should wear eye protection when appropriate. Do not taste, drink, or eat anything used in the experiment, even when individual ingredients began as food. Avoid splashing eyes or skin, wash hands, and wipe spills promptly. For more detailed procedures and safety tips, consult the American Chemical Society’s red cabbage activity and the Royal Society of Chemistry’s red cabbage rainbow lesson.
Frequently asked questions
Can I use green cabbage instead? It lacks the same amount of purple anthocyanin pigment, making the visible changes much harder to demonstrate.
Does this give an exact pH reading? No. A homemade indicator gives a useful approximate color range; calibrated pH paper or a meter is better for precise numbers.
How long does the color last? It can fade or change during storage, so make fresh indicator for the clearest comparisons.
Can I pour all the cups together at the end? The safest approach is to use only food-grade mild samples and dispose of small quantities according to local guidance; never mix unknown household chemicals.
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