CAPS Topic: Acids and Bases · Term 3
An acid-base indicator is a weak acid or weak base that changes colour depending on the pH of the solution it is placed in.
Indicators work because the acid form (HIn) and conjugate base form (In⁻) have different colours. When the pH changes, the equilibrium shifts, causing a visible colour change.
In acid (high [H⁺]), equilibrium shifts left → colour A appears.
In base (low [H⁺]), equilibrium shifts right → colour B appears.
pH = −log[H⁺]. The scale runs from 0 (strongly acidic) to 14 (strongly basic/alkaline).
pH < 7. Donate H⁺ (proton donors). Examples: HCl, H₂SO₄, CH₃COOH. Turn blue litmus red.
pH = 7. Equal concentrations of H⁺ and OH⁻. Example: pure water at 25°C.
pH > 7. Accept H⁺ (proton acceptors). Examples: NaOH, NH₃, Na₂CO₃. Turn red litmus blue.
Adjust the pH and select an indicator. Observe the colour change in real time.
Common acid-base indicators used in Grade 11 Physical Sciences (CAPS).
For a titration, the indicator must change colour within the steep portion of the titration curve (near the equivalence point).
| Titration Type | Equivalence Point pH | Recommended Indicator |
|---|---|---|
| Strong acid + Strong base | ≈ 7 | Bromothymol blue, Phenolphthalein |
| Weak acid + Strong base | > 7 | Phenolphthalein |
| Strong acid + Weak base | < 7 | Methyl orange, Methyl red |
10 questions covering CAPS Grade 11 acid-base indicator content.