- Age:4+
- Time:30 min
- Difficulty:Easy
- Mess level:Medium (cabbage juice stains)
- Supervision:Yes
🧒 In one sentence
Red cabbage juice contains a colour that turns pink in sour liquids and blue or green in soapy, bitter ones, so it can tell you what is in a potion.
Ah, so you want to be a master of potions? Well, then pay the close attention, because today we are sharing one of our favorite recipes. A color-changing potion! It's part of our autumn Pumpkins & Potions lab. The secret ingredient is surprisingly ordinary: red cabbage. Its purple juice turns pink, blue or green depending on what you add, and the colour tells you something about each liquid. This is the same idea scientists use in the lab with pH indicators, but in our case the indicator comes from the garden. But even if the ingredients are ordinary, the effect is magical. You can make a rainbow of potions, and even switch them back and forth.
Materials
For the potion base:
- 1/4 of a red cabbage
- About 1 litre (4 cups) of hot water
- A knife and a chopping board
- A sieve and a jug or a bottle to hold the juice
Ingredients to test:
- Lemon juice
- White vinegar
- Plain water
- Sparkling water
- Baking soda dissolved in a little water
- Soapy water (a few shavings of bar soap dissolved in a little water)
- Optional: milk, apple juice, clear dish soap, a crushed antacid tablet in water
And:
- 6 or more clear glasses or jars
- Spoons, droppers or small cups
- A small funnel, if you use bottles
- Paper labels and a pen
- Optional: a black pot as a cauldron, plastic spiders, a witch's hat
Everything for the potion lab: a red cabbage, a few kitchen liquids to test and some little bottles.
How to make the potions
- Adult step: chop the cabbage into small pieces, put them in a bowl and cover with hot water. Leave for 10 to 15 minutes, until the water is deep purple. (Or blend the cabbage with water and strain it.)
- Strain the juice into a jug and let it cool. This purple liquid is your potion base.
- Pour the potion base into the glasses, about halfway.
- Before adding anything, predict: which ingredient will turn its potion pink? Which will turn it blue or green? Write the guesses on the labels.
- Add a spoonful of a different ingredient to each glass and stir. Watch the colour change.
- Line the glasses up from pinkest to greenest. You've made a pH rainbow.
👨👧 Adult supervision needed
Everything here is food-safe, but the potions are for looking, not drinking. Never test bleach, drain cleaner, dishwasher tablets or other strong cleaners: they can burn skin and eyes. Cabbage juice stains clothes and wooden tables, so use a tray and old shirts.
What colour will each potion turn?
| What you add | Acid or base? | Typical colour |
|---|---|---|
| Lemon juice | Acid (very sour) | Bright pink or red |
| Vinegar | Acid | Pink |
| Sparkling water | Weak acid | Pinkish purple |
| Plain water | Neutral | Purple, no change |
| Baking soda | Weak base | Blue |
| Soapy water | Weak base (a little stronger) | Blue to blue-green |
Clear dish soap can surprise you. Many are close to neutral, so the potion barely changes. That's a good lesson in chemistry: you can't tell what a liquid is just by how it looks.
The grand finale: a potion that changes back
- Take a pink potion (with vinegar) and add baking soda water a little at a time. Watch it turn purple, then blue.
- Now squeeze lemon juice into a blue potion. Back to pink!
- For a witch's brew, put a spoonful of baking soda in an empty glass, pour in some potion base (it turns blue-green), then add vinegar. It fizzes up and turns pink at the same time.
You have a blue potion made with baking soda. What happens when you pour in vinegar?
Make your prediction, then tap an answer to check!
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See what you’ll get →What's happening? Acids, bases and pH
Red cabbage gets its colour from natural pigments called (Natural pigments that make red cabbage, blueberries, red grapes and many autumn leaves red, purple or blue.). The same family of pigments colours blueberries, red grapes and many autumn leaves.
Anthocyanin molecules change shape slightly depending on how acidic or basic the liquid around them is, and each shape reflects a different colour of light. So the colour of the potion works like a code:
- Acids (lemon juice, vinegar) turn it pink or red. Acids taste sour.
- Neutral liquids (water) leave it purple.
- Bases (baking soda, antacids, soap) turn it blue, then green, and very strong bases even yellow. Bases taste bitter and feel slippery.
Scientists measure this on the pH scale, from 0 to 14. Pure water sits in the middle at 7. Lower numbers are acids, and the lower the number, the stronger the acid. Higher numbers are bases. Your row of potions, from pink to green, is a homemade pH scale.
🔬 Make it a real experiment
Make pH paper: soak strips of coffee filter or kitchen paper in strong cabbage juice and let them dry. Then become a potion detective: put a strip into mystery liquids from the kitchen (apple juice, milk, tea, a dissolved antacid) and sort them into acids, neutrals and bases. Record your results in a table. Do any results surprise you?
✂️ Free printable
Pumpkins & Potions lab pack
4 pages · 694 KB
A potion record sheet with a pH rainbow to colour in, a slime and oobleck lab, an autumn colour hunt and 10 cut-out potion labels for your jars.
Make it spooky
- Give every potion a name and a label: "Dragon tears" (lemon), "Bat wing juice" (vinegar), "Swamp water" (baking soda), "Midnight brew" (plain).
- Pour the potion base into a black pot and let children "cast spells" by adding drops with a dropper.
- Freeze the cabbage juice in an ice cube tray. Drop the purple cubes into vinegar and baking soda water and watch the colour spread as they melt.
- Write a secret message on paper with a cotton bud dipped in baking soda water. Once it dries, brush it with cabbage juice and the message appears in blue-green.
Questions to ask
- Which liquids do you think will turn pink? Why?
- What happens if we mix a pink potion with a green one?
- Can you make all the colours of the rainbow by using different amounts?
- Lemons are sour. Is everything sour an acid?
Love colour chemistry? See what happens when soap meets milk in our colourful milk experiment, or mix your own colours in the colour mixing lab.
Key takeaways
- Red cabbage contains anthocyanins, natural pigments that change colour with pH.
- Acids like lemon juice and vinegar turn the potion pink or red; bases like baking soda turn it blue or green; water leaves it purple.
- The pH scale runs from 0 to 14, with 7 as neutral, below 7 acidic and above 7 basic.
- Mixing an acid and a base neutralises them, so a potion can switch colour back and forth.
- The potions are food-based but not for drinking, and strong household cleaners should never be part of the test.
Frequently Asked Questions
How do you make red cabbage indicator?
Chop a quarter of a red cabbage, cover it with hot water and leave it for 10 to 15 minutes until the water is deep purple. Strain out the cabbage and let the liquid cool. You can also blend the cabbage with water and strain it.
Why does red cabbage juice change colour?
It contains anthocyanins, pigments whose molecules change shape depending on how acidic or basic their surroundings are. Each shape reflects a different colour, so acids make it pink and bases make it blue or green.
What colour does red cabbage juice turn with vinegar and with baking soda?
With vinegar it turns pink, because vinegar is an acid. With baking soda it turns blue or blue-green, because baking soda is a weak base.
How long does red cabbage juice last?
It keeps for about two days in a closed jar in the fridge. For longer, freeze it in an ice cube tray and thaw cubes as you need them. Throw it away if it starts to smell.
Is this experiment safe for young children?
Yes, with an adult nearby. The ingredients are all food-safe, but agree that the potions are not for drinking, and keep strong cleaners out of the experiment. An adult should handle the knife and the hot water.
If you liked the colour-changing potions and want more acid and base chemistry, soak an egg in vinegar until its shell disappears in our bouncy egg experiment, or mix baking soda and vinegar into an erupting homemade volcano. Curious why a cut apple turns brown, and why lemon juice stops it? Find out in our apple oxidation experiment.
Happy brewing!
