A pH indicator is a substance that has a different colour when added to acid or alkali. In this experiment a pH indicator is made from red cabbage.

Class practical

pH indicator is a substance which has one colour when added to an acidic solution and a different colour when added to an alkaline solution. In this experiment pupils make an indicator from red cabbage.

Lesson organisation

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.

Apparatus Chemicals

Eye protection for all

Each working group will require

Beaker (250 cm3)

Bunsen burner

Tripod

Gauze

Heat resistant mat

Test-tubes, 3 (Note 1)

Test-tube rack

Dropper pipette

Several pieces of red cabbage

Access to:

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 cm3) 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 cm3 are ideal.

 

Procedure

a Boil about 50 cm3 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.

Teaching notes

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 

Credits

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