Red cabbage

Public summary: 

We use a little red cabbage juice to find out more about acids and bases in the world around us.

Making a natural pH indicator.
Useful information
Kit List: 

Red cabbage
Rolling pin or other mashing device
Large bowl
Vinegar or lemon juice
Absorbent white paper - scientific/coffee filter paper are good, kitchen roll less good
Litmus paper

Packing Away: 

Wash down surfaces.
Wash up bowl/knife/etc.
Cabbage waste can be binned.

Frequency of use: 

Cabbage juice is used as a pH indicator to demonstrate the concept of pH. Paper dipped in cabbage juice will change colours when dipped in an acid (vinegar/lemon juice) or base (bicarb).

Tips for demonstrating: If the kids want to, let them take home the pH paper!

Before Event:
1) Cut up half a head of red cabbage.
2) Crush it using a rolling pin. Put crushed cabbage in a bowl/cup with water and mix. Strain the liquid into another cup, and you should have a bluish/dark purple liquid – this is your natural indicator. When added to acids it should go pink, and in alkalis or bases it should go blue/green.
3) Set up cups with cabbage juice, lemon juice/vinegar, bicarb in water and water.

At Event:
4) If the cabbage still needs to be crushed/strained into a cup, ask the kids to help you.
5) Dip a strip of paper into the juice and explain that this juice will act as a pH indicator – in other words it will tell us whether something is acidic or basic.
6) Ask them if they can name some acids/bases. (Lemon juice, bicarb, vinegar, orange juice, etc)
7) Ask them if they know what pH is?
-tailor your explanation to the age of the child
-pH scale is a logarithmic scale defined by pH = -log10[H+] – for older kids
-pH is a number that tells you how acidic or basic something is [neutral substances, such as water, have a pH of seven, acids (vinegar/lemon juice) have a pH of less than seven, and bases (sodium bicarbonate) have a pH of more than seven.]
8) Explain that the cabbage juice will turn blue/green when dipped in a base and pink when dipped in an acid.
9) Show them your “mystery” liquids (vinegar, water and bicarb/water) – ask them to use the indicator paper to determine which one is an acid/base.
10) Discuss why the indicator paper doesn't change colour when dipped in water as it's neither an acid nor a base - explain the concept of neutral substances.
11) You can also use a pipette to add a couple of drops of acidic/basic solution to the cup with cabbage juice in to make that change colour.

1) Structure of atoms
-get kids to simulate and atom by having one kid be the nucleus and the others be electrons that orbit the nucleus
-talk about how electrons have negative charge and protons in the nucleus have positive charge
-can they explain how the electrons orbit rather than flying off? Get them to think about magnets attracting each other if struggling
-talk about how acids are solutions with free protons and how this allows most indicators to gain a proton/H+ in acidic conditions to change colours (contrast with how red cabbage loses an OH group as described below)
2)How can the cabbage juice act as an indicator?
-red cabbage contains coloured pigments called “anthocyanins” which have antioxidant properties
-in acidic conditions they lose an –OH group, and gain it in basic conditions
-most indicators gain an H+ in acidic conditions and lose in again in basic conditions
- This change in the physical structure will change the wavelength of light reflected off it, and so it changes colour

Risk Assessment
Date risk assesment last checked: 
Tue, 07/01/2020
Risk assesment checked by: 
Holly Smith
Date risk assesment double checked: 
Fri, 10/01/2020
Risk assesment double-checked by: 
Risk Assessment: 

Cut up red cabbage before the event. Then mash, e.g. using a half rolling pin, to give pH indicator solution. Paint onto paper and observe colour change when lemon juice/vinegar or bicarb solution is added.


Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Knife Sharp knives may cause cuts. All 3 3 9 Only demonstrators to use knives to cut cabbage, which should be done before the event starts. Knives to be kept concealed and out of reach for the duration of the experiment. Knife to be sheathed when not being used.
Call first aider in case of injury.
2 3 6
Lemon juice/vinegar/bicarb Lemon juice/vinegar and bicarbonate of soda irritant to eyes All 2 3 6 Only use small amounts, avoid pouring out more reagents while demonstrating.
If in contact with eyes, wash with emergency eye wash provided if trained and confident to do so. Call first aider if necessary.
1 3 3
Lemon juice/vinegar/bicarb Children eating/drinking bicarbonate, lemon juice or vinegar. Public 3 1 3 Don't leave children with experiment unsupervised. Put all unnecessary reagents back in the box and always leave the experiment safely packed away when closed.
If ingested, advise parents that reactants are all edible, but to seek medical attention if child is feeling unwell as reactants may have been in box for unknown amount of time.
2 1 2
Moving children Children bump into each other or trip Public 4 1 4 Tell children to be sensible and don't run. Call a first aider in case of injury. 3 1 3
Publicity photo: