Making a pH indicator
is a substance which has one colour when added to an pH indicator and a different colour when added to an acidic solution . In this experiment pupils make an indicator from red cabbage. alkaline solution
The experiment is in two parts. The first part involves boiling some red cabbage in water. In the second part the students test their indicator. Between the two parts the mixture must be allowed to cool. The first part takes about 10 to 15 minutes. The cooling takes about 15 minutes and the testing less than 5 minutes.
The cooling period could be used as an opportunity to discuss the background to the experiment – see Teaching notes below.
Eye protection for all
Each working group will require
Beaker (250 cm
Heat resistant mat
Test-tubes, 3 (Note 1)
Several pieces of red cabbage
Dilute hydrochloric acid, 0.01 M
Sodium hydroxide solution, 0.01 M
De-ionised or distilled water
Refer to Health & Safety and Technical notes section below for additional information.
Health & Safety and Technical notes
Read our standard health & safety guidance
Wear eye protection throughout. Consider clamping the beaker.
Each group of students will need access to the hydrochloric acid and sodium hydroxide solutions, and to de-ionised or distilled water. Provide all three in similar containers labelled ‘Acid’, ‘Alkali’ and ‘Water’. Dropper bottles are ideal. Alternatively small beakers (100 cm
3) with dropper pipettes could be used. Students need to be able to pour the acid and alkali solutions easily and safely into test-tubes.
Dilute hydrochloric acid, HCl(aq) - see CLEAPSS Hazcard.
Sodium hydroxide solution, NaOH(aq) - see CLEAPSS Hazcard.
1 Small test-tubes of capacity about 10 cm 3 are ideal.
a Boil about 50 cm 3 of water in a beaker.
b Add 3 or 4 small (5 cm) pieces of red cabbage to the boiling water.
c Continue to boil the red cabbage in the water for about 5 minutes. The water should turn blue or green.
d Turn off the Bunsen burner and allow the beaker to cool for a few minutes.
e Place 3 test-tubes in a test-tube rack. Half-fill one of the test-tubes with acid, one with alkali, and one with distilled or de-ionised water. Label the test-tubes.
f Use a dropper pipette to add a few drops of the cabbage solution to each test-tube. Note the colour of the cabbage solution in each of the three test-tubes.
Discussion points could include any or all of the following.
Many plant colouring materials in berries, leaves and petals act as indicators.
Some of these will not dissolve in water easily. A solvent other than water (e.g. ethanol) could be used, but it may be flammable. Discuss how the risk of fire can be reduced by using a beaker of hot water to heat the mixture.
Possible variations on this experiment might include using beetroot, blackberries, raspberries, copper beech leaves, or onion skins in place of the red cabbage.
Health & Safety checked, July 2016
This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.
© Nuffield Foundation and the Royal Society of Chemistry
Page last updated July 2016