Acids bases complex formation dying indicators pigments
Age Group
16 17 18
Expected time
1 period
The (un)reliable indicator.PDF
beaker symbol

The (un)reliable indicator

What’s the mystery? Students are testing different substances for the pH value. Some students are testing crystal (alum) deodorants using a red cabbage indicator and a universal indicator. The universal indicator suggests that the deodorant reacts as an acid, whereas the red cabbage suggests that the deodorant reacts as a base. What is happening here?
Domain(s) Chemistry
Subdomain keywords Acids, bases, indicators, complex formation, pigments, dying.
Age group On a submicroscopic level: 16 to 18 years old.On a phenomenological level: 12 years and older.
Expected time for the mystery Approximate time for teacher preparation: 30 min.Approximate time in classroom: two to three individual 50-min lessons.
Safety/supervision There are no particular safety restrictions for the suggested materials.Disclaimer: the authors of this teaching material will not be held responsible for any injury or damage to persons or properties that might occur in its use.
Preparation and list of materials Several substances to test whether they are basic or acidic, e.g. lemon juice, vinegar, soap, baking powder, water, and crystal (alum) deodorants. These cause the mysterySubstances to explore the mystery: iron salts, aluminium salts, several deodorants (some with and some without aluminium salts, either liquid or solid), alum, red roses, raspberries, etc. Put the crystal deodorants in a beaker with water some minutes before using the solutionRed cabbage indicator, universal indicator, indicator paper, pH-metre, etc.Charts showing the colours of the red cabbage indicator and the universal indicator at different pH values, e.g. paper, FriXtion pen per group of six students, hair drier, liquid nitrogen.
Learning objectives Students can describe how a red cabbage indicator normally works. They learn details about anthocyanidins and realise that they form coordination complexes with aluminium and iron ions. Students learn to be sceptical and reflective about outcomes. Students learn to use different measuring devices to be sure about the outcome.