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Description

Our question Can purple cabbage water tell us whether a substance is acidic, neutral or alkaline?

Science rules
Do not taste anything. Keep fingers, pipettes and cups away from your mouth.
Only test the substances your leader gives you. Never mix household cleaners.
Wear eye protection and an apron or old top. Cabbage water and food liquids can stain.
Add only a few drops at a time. Keep cups on the tray.
Tell a leader straight away if there is a spill or anything gets in your eyes.
Wash your hands when you finish.

Resources

For each group
Clear cups or test tubes; tray; pipettes or clean teaspoons; white paper; pencil; eye protection.

Purple indicator; pre prepared by leader before the session
Cooled and strained red cabbage indicator in a labelled jug or dropper bottle.

Suggested test samples
Water; white vinegar; lemon juice or dissolved citric acid; bicarbonate of soda solution; mild washing-up-liquid solution.

For recording
The prediction and results table, or a separate worksheet.

Instructions

Leader preparation
Chop about one quarter of a red cabbage into small pieces.
Place it in a heatproof bowl. Cover with freshly boiled water. Leave for 15–20 minutes until the water is deep purple.
Strain out the cabbage. Allow the purple liquid to cool completely before children use it.
Adult only Handle the knife, boiling water and hot bowl. Do not use bleach, drain cleaner, dishwasher products, concentrated cleaners or unknown chemicals.

Work in pairs or a small group. One person can be the dropper, one the observer and one the recorder. Swap jobs if you want to.

1. Set up Put a tray on the table. Place one clear cup on a piece of white paper.
2. Add the indicator Pour in enough purple cabbage indicator to cover the bottom of the cup — about 2 tablespoons.
3. Choose one sample Read its label. Do not pick up a different sample until this test is finished.
4.Make a prediction Do you think it is acidic, neutral or alkaline? Write or circle your guess before testing.
5. Add the sample Use a clean pipette or spoon. Add 5 drops to the purple indicator.
6. Watch Gently swirl the cup. Do not shake it. Wait about 10 seconds and look for a colour change.
7. Add a little more if needed If the colour has hardly changed, add another 5 drops. Stop after about 20 drops.
8. Compare Use the colour guide below. Decide whether your sample is acidic, neutral or alkaline.
9. Record Write the colour you saw, or colour the result box. Then write the type of substance.
10. Start the next test Use a fresh cup and a clean pipette or spoon. Never pour a tested mixture back into a sample cup.

Simple colour guide
Pink or red; Acidic
Purple; Neutral
Blue; Weak alkali
Blue green; Alkaline
Yellow green; Stronger alkali

Important. Colours are approximate. The amount of sample, the strength of the mixture and lighting can change the shade. The supplied household samples should not produce the extreme ends of a full laboratory pH scale.

Think like a scientist
Which sample caused the biggest colour change?
Did two acidic samples make exactly the same shade? Why might they differ?
Why must we use a fresh cup and clean pipette for every test?
What could we keep the same to make this a fair comparison?

Finish and tidy Follow your leader’s instructions for disposal. Put used equipment in the washing-up area, wipe the tray and table, wash your hands and check that everyone has recorded their results.

How does it work
Red cabbage contains natural colour chemicals called anthocyanins. Their shape changes when the acidity of a liquid changes. That shape change makes them absorb and reflect light differently, so we see a different colour.
The pH scale describes how acidic or alkaline a water-based liquid is. A pH below 7 is acidic, pH 7 is neutral, and a pH above 7 is alkaline. Red cabbage gives us an estimate, not an exact pH number.

Tags

• chemical reaction
• experiment badge
• investigate
• Ph indicator
• scientist badge

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