ChemixaLEARN CHEMISTRY

Red cabbage pH indicator

Objective: prepare a natural acid-base indicator from red cabbage and use it to classify common household liquids as acidic, neutral, or basic. You will observe real color changes, record them, and connect each color to the pH range of the liquid tested.

Safety:

Adult supervision is required: an adult must handle all boiling water and the hot pot. Let the cabbage liquid cool before touching it. Do not drink the indicator or the test mixtures. Rinse skin if the colored liquid stains.

Materials

  • Half a head of red cabbage, chopped
  • Water
  • Pot and stove (adult use only)
  • Strainer or colander
  • Clear cups or glasses
  • White vinegar
  • Baking soda dissolved in water
  • Lemon juice, soap water, and other test liquids

Procedure

1

Prepare the cabbage

Chop half a red cabbage into small pieces and place them in a pot.

2

Boil to extract the pigment

An adult adds enough water to cover the cabbage and brings it to a gentle boil for about 10 minutes, until the water turns deep purple.

3

Strain and cool

An adult strains the hot liquid into a bowl or jar through a colander, then lets it cool to room temperature.

4

Set up test cups

Pour small amounts of the cooled purple liquid into several clear cups.

5

Add test liquids

Add a spoonful of a different test liquid to each cup: vinegar, lemon juice, baking soda solution, soap water, and plain water as a control.

6

Record and rank

Record the color in each cup and arrange your liquids from most acidic to most basic.

What to observe

  • The strained cabbage liquid is deep purple or blue.

  • Vinegar and lemon juice turn the indicator pink or red.

  • Baking soda solution turns it green or blue-green.

  • Plain water leaves it purple.

Why it happens

Red cabbage contains anthocyanin pigments, the same family of molecules that color blueberries and autumn leaves. Anthocyanins change their molecular structure as hydronium ion concentration changes, and each structure absorbs light differently, which is why you see different colors. In strongly acidic solutions the pigment turns red or pink; near neutral it is purple; in basic solutions it shifts through blue to green and even yellow. This is exactly how commercial indicators like litmus work: a molecule with an acid-base equilibrium whose two forms have different colors. The cabbage juice is less precise than a pH meter, but it reliably sorts household liquids into acidic, neutral, and basic.

Try variations

  • Test rainwater, milk, or antacid tablets dissolved in water.

  • Dilute the indicator and see whether the colors get harder to read.

  • Compare your cabbage colors with pH paper on the same liquids.