Science 7–10 · Years 9–10
Red cabbage indicator: reading acids and bases by colour
Reactions
The idea
An indicator is a substance whose colour depends on how acidic or basic a solution is, so it turns the invisible property of pH into something a learner can see and rank.
Safety card
Hazards
- sodium hydroxide 0.1 mol/L and hydrochloric acid 0.1 mol/L are irritants
- hot water while making the indicator
Controls
- eye protection
- adult handles the boiling step
- solutions diluted and washed to the sink
Note
NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.
What you need
- red cabbage, about 50 g chopped, 250 mL beaker, 100 mL water, tripod, gauze, Bunsen burner or kettle (adult)
- filter funnel and paper, dropping pipettes, spotting tile or six test tubes in a rack
- test solutions 2 mL each: hydrochloric acid 0.1 mol/L, vinegar, lemon juice, tap water, sodium hydrogencarbonate solution, sodium hydroxide 0.1 mol/L
- universal indicator paper or a pH meter to check the ranking
How to do it
- Boil the chopped cabbage in 100 mL of water for 5 minutes over the Bunsen burner, or pour 100 mL of just-boiled water from the kettle over it and leave it until the water is strongly coloured (adult at the heat source); cool and filter; the purple filtrate is the indicator.
- Put 2 mL of each test solution in its own well or tube and add 5 drops of indicator to each.
- Record each colour and arrange the solutions from most acidic (red) through neutral (purple) to most basic (green then yellow).
- Check the ranking with universal indicator paper or a pH meter and record the pH values beside the colours.
What you should see
Hydrochloric acid 0.1 mol/L (pH 1.0) and lemon juice turn the indicator red; vinegar (about pH 2.5 for 4 per cent acetic acid) pink-red; tap water stays purple; sodium hydrogencarbonate turns blue-green; sodium hydroxide 0.1 mol/L (pH 13.0) turns green to yellow. The learner knows it worked when the colour order matches the pH-paper order without any swaps.
What changes
- What you change
- test solution
- What you measure
- indicator colour and pH
- What you keep the same
- volume of solution
- number of indicator drops
- same batch of indicator
- temperature
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Every acid is dangerous and every base is safe.
- Neutral means the solution contains nothing.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-RXN-01SC5-WS-05
- Australian Curriculum v9AC9S9I04AC9S10I04
Sources
The pages the author read to write this activity.