Red Cabbage pH Indicator Experiment: Test Acids and Bases with Kitchen Liquids

Few experiments are as colourful as the red cabbage pH indicator. Add cabbage juice to vinegar and it turns pink. Add it to baking soda solution and it turns blue-green. It’s a perfect first look at acids and bases, and it uses things from your kitchen.

The science in one paragraph

Red (purple) cabbage contains pigments called anthocyanins. These molecules change shape, and therefore colour, depending on how acidic or basic the liquid around them is. In strong acids they look pink or red, near neutral they look purple, and in bases they shift through blue and green to yellow. A chemical that changes colour like this is called an indicator.

Make the indicator (adult step)

1. Chop about a quarter of a red cabbage and put it in a heat-safe bowl.
2. A grown-up pours very hot water from a kettle over it to cover and lets it stand for 10 to 15 minutes.
3. Strain off the purple liquid and let it cool completely.

The liquid will be deep purple. Keep it in the fridge and use it within a day or two.

What to test

Use small clear cups or a white egg carton so the colours show. Good liquids to try:

• White vinegar and lemon juice (acids)
• Clear lemon-lime pop or sparkling water (mildly acidic)
• Milk and tap water (close to neutral)
• Baking soda dissolved in water (a base)
• Clear dish soap in water (a base)

Don’t test strong cleaners, bleach or drain cleaner. They are hazardous, and mixing household chemicals can release harmful gases. Goggles on, and everything stays out of mouths and eyes.

The method

1. Put a few spoonfuls of each liquid in its own cup, labelled.
2. Add the same amount of cabbage juice to each, about the same as the liquid.
3. Look at the colours. Make a colour scale, from pinkest to greenest.
4. Sort the liquids: acids, near neutral, bases.

Make it a real investigation

• Predict first. Which liquids do you think will be acids? Why?
• Mystery liquids. Have a grown-up prepare two unlabelled cups, one of vinegar water and one of baking soda water. Use your colour scale to identify them.
• Cancel it out. Add baking soda solution to a pink acid cup a teaspoon at a time. The colour moves from pink through purple to blue. Acid and base are cancelling each other out. This is a neutralization reaction, and the fizzing you may see is carbon dioxide gas.

Our lesson Can red cabbage tell acid from base? takes students through all of this, ending with the neutralization demonstration.

Why this experiment works well with kids

It has an immediate, visible result, so no one waits for something to happen. It also makes a useful point: an acid is not always dangerous (lemon juice is an acid), and a base is not always safe (some are strongly corrosive). The colour tells you where a liquid sits on a scale, not whether it is safe to touch.

Common mix-ups

• “Acids are always dangerous.” Vinegar and citrus fruit are acids. Strength matters.
• “Cabbage juice makes liquids acidic or basic.” It only shows what’s already there.
• “Neutral means nothing is in it.” Neutral means neither acidic nor basic, like pure water.

Next steps

You’ll find this activity in the Kitchen Chemistry unit for Grades 7 and 8, along with lessons on new substances and where mass goes in a reaction. Younger children can try the family version of kitchen chemistry in Kitchen Chemistry for Families. Sign up to get three lessons free.