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Halloween Potion: Red Cabbage Indicator Experiment

A Spooky Potion for Halloween

This Halloween, turn your kitchen into a witch's laboratory with a potion that changes colour before your eyes. One deep purple brew becomes vampire red, witch's pink and ghostly blue, then finishes with a fizzing eruption.

The secret ingredient is a red cabbage indicator, a natural dye that reacts to acids and alkalis. In this guide, APC Pure will show you how to make it at home with a few simple ingredients.

What You Will Need

Red cabbage: A quarter of one is plenty.

Distilled Water: About 1 litre. Distilled water gives the truest purple, as hard tap water can be slightly alkaline and tint the potion blue.

Citric Acid: 1 teaspoon.

Acetic Acid 5% Solution: 2 tablespoons.

Sodium Hydrogen Carbonate: 1 teaspoon, also known as bicarbonate of soda.

You will also need four clear glasses or jars, a heatproof jug, a sieve, a tray to catch spills and a pair of gloves.

Brewing the Potion Base

1. Chop the red cabbage into small pieces and place them in a heatproof jug.

2. An adult should pour over enough just-boiled distilled water to cover the cabbage.

3. Leave it for 20 to 30 minutes, until the water turns a deep purple.

4. Strain out the cabbage and let the liquid cool. This purple liquid is your potion base.

Casting the Spells

Stand four glasses on a tray and pour about 100ml of potion base into each one. Leave the first glass as it is, so you have a purple potion to compare against.

Glass 2, Vampire Red: Stir in 1 teaspoon of citric acid. The potion turns a bright pinky red.

Glass 3, Witch's Pink: Add 2 tablespoons of acetic acid. The potion turns a softer pink.

Glass 4, Ghostly Blue: Stir in 1 teaspoon of bicarbonate of soda. The potion turns blue or blue-green.

The Finale

For the finale, slowly pour the Vampire Red potion into the Ghostly Blue glass.

The potion will bubble and fizz as the acid meets the bicarbonate of soda, and the colour will change from blue back to pink. Keep the glass on the tray, as it may overflow.

How the Potion Changes Colour

Red cabbage contains natural pigments called anthocyanins, which change colour depending on whether a liquid is acidic or alkaline. Citric acid and acetic acid are acids, so they turn the potion red and pink. Bicarbonate of soda is a mild alkali, so it turns the potion blue.

In the finale, the acid and bicarbonate of soda react to produce carbon dioxide gas, which creates the fizz. The acid also cancels out the alkali, which is why the colour changes back.

Try our DIY Red Cabbage pH Paper, or read how phenolphthalein changes colour from acid to alkali.

Staying Safe

This experiment should always be supervised by an adult. An adult should handle the boiling water, and children should wear eye protection, as citric acid and acetic acid can sting if they get into the eyes. The potions are not for drinking, and no other cleaning products should ever be added.

When you have finished, pour the potions down the sink with plenty of water and rinse the glasses.