Using Red Cabbage Indicator
The pH scale is used to measure the acidity or alkalinity of a substance.
Strong acids have a pH of 0 – 2.
Weak acids have a pH of 3 – 6.
A pH of 7 is neutral.
Weak alkalis have a pH of 8 – 11.
Strong alkalis have a pH of 12 – 14.
We can use an indicator to tell whether a substance is acid, alkaline or neutral. The indicator turns
a range of colours, depending on the pH of the substance.
The pH scale for red cabbage indicator is shown below:
0
1
2
3
4
increasingly acidic
5
6
7
8
neutral
9
10
11
12
13
14
increasingly alkaline
If added to a neutral substance, the indicator will be purple. If added to an acidic substance
with a pH of 2, it will turn red. If added to an alkaline substance with a pH of 11, it will turn
green.
You can use the indicator to test the pH of household products, food and drink in two ways:
1. Add some of the substance to be tested to a glass. Add some red cabbage indicator and stir.
Observe the colour change. Use a separate glass for each substance or rinse out with water
between uses. This method works best with clear, white or pale substances.
2. Dip a red cabbage indicator strip (made by soaking strips of fabric in the indicator and
leaving them to dry) into a liquid sample. Remove the strip and observe the colour change.
This method can work with some darker substances if they can be wiped cleanly off the
indicator strip.
Safety Information
Acids and alkalis can cause irritation to skin, eyes and respiratory organs
and are harmful if swallowed. Children should be supervised at all times
with these substances.
Eye protection should be worn during the investigation.
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Using Red Cabbage Indicator
Record the substances that you test and the colour change in the table below. Use the pH scale to
find their pH and decide whether they are an acid, an alkali or a neutral substance.
Suggested Substances
- lemonade
- toothpaste
- bleach
- handwash
- lemon juice
- vinegar
- bicarbonate of soda
- washing powder
- orange juice
- cream of tartar
- cream household cleaner
Results Table
Substance
Colour
pH
Acid, Alkali or
Neutral?
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