Making a pH indicator

Read our standard health & safety guidance

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 and chemicals

Eye protection for all

Each working group will require:

Beaker (250 cm³)
Bunsen burner
Tripod
Gauze
Heat resistant mat
Test-tubes, 3 (see note 1)
Test-tube rack
Dropper pipette

Several pieces of red cabbage

Access to (see notes 2 and 3):

Dilute hydrochloric acid, 0.01 mol dm^-3 (Low hazard at this concentration)
Sodium hydroxide solution 0.01 mol dm^-3 (Low hazard at this concentration)
De-ionised or distilled water

Technical notes

Dilute hydrochloric acid, 0.01 mol dm^-3 (Low hazard at concentration used). Refer to CLEAPSS Hazcard 47A
Sodium hydroxide solution 0.01 mol dm^-3 (Low hazard at concentration used). Refer to CLEAPSS Hazcard 91

1 Small test-tubes of capacity about 10 cm³ are ideal.

2 Each group of students will need access to the hydrochloric acid and sodium hydroxide solutions. 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.

3 Provide similar containers for de-ionised or distilled water. Label the containers ‘Acid’, ‘Alkali’ and ‘Water’.

Procedure

SAFETY: Wear eye protection throughout. Consider clamping the beaker.

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 and Safety checked, November 2006