Homemade pH Indicator from Red Cabbage

Red cabbage contains a pigment called anthocyanin that changes colour depending on the pH of its environment. Acidic solutions turn it pink-red, neutral solutions keep it purple, and alkaline solutions turn it blue-green or yellow. This makes it a perfect — and completely edible — natural pH indicator. The project teaches about the pH scale, acids, bases, and light-absorbing molecules.

What You’ll Need

• ¼ head of red cabbage
• Water (500 ml)
• Blender or knife and pot for boiling
• Strainer or coffee filter
• Clear glasses or jars (one per test liquid)
• Test liquids: lemon juice, vinegar, milk, water, baking soda solution, soap, antacid solution
• Labels and pen

Step-by-Step Instructions

1. Extract the indicator: Chop the cabbage into rough pieces and blend with 200 ml of water until smooth. Strain through a coffee filter into a jug. Alternatively, boil chopped cabbage in 500 ml water for 10 minutes, then strain. The liquid should be a deep purple-blue.
2. Prepare test jars: Pour about 50 ml of the cabbage indicator into each clear glass and label them for each test liquid.
3. Add test substances: Add a small amount (1–2 tablespoons) of each test liquid to the corresponding indicator jar. Stir gently and observe.
4. Record results: Note the colour change for each substance. Lemon juice → bright pink; vinegar → red-pink; water → purple (no change); baking soda solution → blue-green; soap → green.
5. Make a pH scale chart: Arrange the glasses in order from most acidic to most alkaline and photograph or sketch the colour gradient.

Safety Notes

• Avoid mixing unknown chemicals or mixing strong acids with strong bases in the same container.
• Bleach (sodium hypochlorite) destroys the indicator and should not be used with children.
• Hands and clothing will stain — wear an apron and gloves.

Tips & Troubleshooting

• Freeze leftover indicator as ice cubes for future experiments — it keeps well frozen.
• Use a white background behind the glasses to make colour differences more visible in photos.
• Antacid tablets dissolved in water show a striking green-yellow result, great for demonstrating what happens to stomach acid.