Teaching acids and bases post-16

Indicators and titrations

Indicators are weak acids in which the dissociated form (HIn) has one colour and its conjugate base (In^-) has a different colour or is colourless. Rainbow fizz is a memorable experiment, often used in lower school chemistry as an introduction to the topic, and provides an opportunity to revisit prior learning as well as a starting point for discussing how indicators work. The Rainbow fizz revisited resources provide all you need to get started and lead on to choosing the right indicator for a titration. The titration screen experiment provides practice with pH curves, equivalence points and indicator selection in context-based problems, including aspirin tablets and hair products.

Acid and base strength

Compare a strong and weak acid by carrying out a series of quick experiments. For example, measure the pH and conductivity of 1M HCl and 1 M CH₃CO₂H, then observe how they each react with magnesium ribbon or marble chips. Comparing the results will help students to understand the differences, especially if you ask them to interpret the reactions of the acids in terms of the number of H⁺ ions present.

You could then go on to explore the equilibria involving carbon dioxide in an aqueous solution, which provides an opportunity to revisit the concept of equilibria before moving on to introduce the acid dissociation constant K_a, which is a mathematical relationship that indicates the degree of dissociation. In this reaction there are several possible equilibria, which makes it quite complicated but allows for interesting discussions and practice writing expressions for K_a. Follow up the theory by determining K_a for a weak acid or base using pH measurements.

The submicroscopic

Modelling the behaviours of acid and alkaline solutions helps students to appreciate what is going on at the molecular level. The PhET simulations pH Scale and Acid–base solutions allow students to investigate a range of different substances, relating the strength of an acid or base to the extent to which it dissociates in water. Students can visually compare the molecules and ions present in a strong acid with a weak acid using the dissociation equation HA + H₂O ⇋ A^- + H₃O⁺ (and similarly for bases). The ability to switch the pH scale between a logarithmic and linear scale highlights numerically why a logarithmic scale is used to measure pH.

Real-life contexts

Relating the chemistry to real-life contexts adds extra interest for students. Examples include:

• Household products such as cleaning products and food (citrus fruits, tomato-based products and processed grain are acidic, while nuts, legumes and vegetables are alkaline).
• Neutralisation has a role to play within agriculture as many cultivated plants require a neutral soil to grow in, but soil pH varies from 3–10.
• The environment – this topic links to air pollution, acid rain and ocean acidification, which many students are passionate about.
• Medicine, for example to diagnose and treat conditions such as heartburn, indigestion or respiratory alkalosis – a condition in which the pH of blood (around 7.4) is above normal, often caused by hyperventilation.
• The three main buffer systems in the body (carbonic acid bicarbonate, phosphate and protein buffer systems), require pH to be controlled so that various biochemical processes can take place, and are especially relevant for those students also studying biology.

Diagnostic probes such as Acid strength can be used to assess your students’ understanding at various points in the topic.

Concept maps are useful tools to assess whether students can make connections between key ideas. The Revising acids activity provides a framework and starting point which can be easily extended to include post-16 concepts by including terms such as equilibria, equilibrium constant, ionic product, log. Students should then be encouraged to find a link between these concepts and add them in the concept map.

The Acids and bases and Inorganics in aqueous solution Starter for ten resources provide a series of short quizzes and activities that can be used as either retrieval practice at the start of a lesson or as an assessment activity. Topics include pH and K_w, Brønsted–Lowry and Lewis bases, acid–base titrations and buffer solutions.