OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 11

✅ Correct Answer

D - Colour stays yellow.

The indicator is already in a yellow solution, meaning it is already in the form A⁻(aq) . Adding an excess of strong alkali ( NaOH ) introduces high concentrations of OH⁻(aq) , which reacts with H⁺ . This shifts the equilibrium further to the right. Since the solution is already fully shifted towards the yellow anion form, adding more alkali produces no further visible change.

💡 Key Knowledge

• Indicators as weak acids: Expressed generally as HA(aq) ⇌ A⁻(aq) + H⁺(aq) .
• Species colours: HA is blue, while its conjugate base A⁻ is yellow.
• Le Chatelier's Principle: If a factor changes in a system at dynamic equilibrium, the position shifts to counteract that change.

🧠 Exam Technique

• Analyze the initial state: The text states the indicator already turns the solution yellow. This means A⁻ is the predominant species present.
• Evaluate the perturbation: Adding excess NaOH provides OH⁻ ions that neutralise H⁺ ( OH⁻ + H⁺ ➔ H₂O ).
• Determine the shift: Removing H⁺ drives the equilibrium to the right. Because the system is already fully in the yellow A⁻ state, no transition back to blue can occur.

❌ Common Errors

• Assuming titration neutrality: Many students see "alkali added" and automatically think of a neutralisation colour change (such as green or blue), ignoring the fact that the solution started yellow.
• Misinterpreting equilibrium shifts: Forgetting that shifting an equilibrium further towards an already dominant product species does not change the visual appearance of the solution.