Preparing a red cabbage indicator and mapping its colours across the pH scale

The idea

An indicator is a weak acid whose acid and base forms differ in colour, so its colour reports the pH and changes reversibly as the pH is moved back and forth.

Safety card

What you need

• Red cabbage, about 100 g, chopped; 250 mL beaker, 100 mL water, hotplate or kettle; sieve
• Reference solutions of known pH: 0.1 mol/L hydrochloric acid (pH 1), 0.01 mol/L (pH 2), 0.001 mol/L (pH 3), buffer pH 4 and pH 7 (commercial), 0.001, 0.01 and 0.1 mol/L sodium hydroxide (pH 11, 12, 13), deionised water
• Test tubes, ten, rack, droppers, white card; calibrated pH meter for checking
• Household samples: vinegar, lemon juice, soda water, bicarbonate solution, washing soda solution, milk

How to do it

1. Simmer the cabbage in 100 mL water for five minutes, cool and strain; the indicator is deep purple.
2. Add 1 mL of indicator to 5 mL of each reference solution and photograph the row against the white card; record the colour for each pH.
3. Measure the pH of each reference with the meter to confirm the labels.
4. Test each household sample with 1 mL indicator and read its pH from the colour chart; check two with the meter.
5. Reversibility: take the pH 1 tube, add sodium hydroxide dropwise until it turns green, then acid until red again; record the colours through the change.
6. Write the equation HIn reversible H+ + In- and explain each colour region as a shift in this equilibrium.

What you should see

The learner's reference row runs from red at pH 1 and 2 through pink-red at pH 3 and 4 and purple at pH 7 to green at pH 11 and 12 and yellow-green at pH 13. Vinegar and lemon juice give the acid colours, milk stays close to the neutral purple, sodium hydrogen carbonate solution turns blue and washing soda green, and the meter confirms the same order. The pH 1 tube cycles red to green to red as base and acid are added, showing the change is reversible.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• Indicators measure how strong an acid is (they report pH, which depends on concentration as well as strength).
• The colour change is a chemical reaction that uses up the indicator (it is a reversible protonation; the indicator is not consumed).
• Water has no pH (deionised water reads near 7 and turns the indicator purple).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-13CH11/12-3
• Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 6, which it allows in any moduleCH11/12-4
• Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-02CH-12WS-03
• Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/chemistry-stage-6-2017
2. www.nsw.gov.au/sites/default/files/noindex/2025-03/chemistry-stage6-syllabus-word.docx
3. curriculum.nsw.edu.au/learning-areas/science/chemistry-11-12-2025/outcomes
4. edu.rsc.org/experiments/making-a-ph-indicator-using-red-cabbage/422.article
5. edu.rsc.org/experiments/testing-the-ph-of-different-solutions/395.article
6. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/chemistry/Chemistry_module_6_IQ3.docx