Lesson overview
Acids, bases and indicators: What does a colour change tell you?
Test four unknown colourless solutions with litmus, methyl orange and thymolphthalein, then with universal indicator paper, and decide which are acids, which is neutral and which is an alkali. Find out what acids, bases and alkalis are and how they react, what the pH scale tells you, and why a strong acid and a weak acid of the same concentration have different pH values.
Suggested sequence
- Lesson 1 · Acid, alkali or neutral? Indicators and the pH scale: litmus, methyl orange, thymolphthalein and universal indicator; acids contain H⁺ ions, alkalis OH⁻ ions; comparing acidity and alkalinity by pH. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · What acids and bases do. What acids and bases do: acids with metals, bases and carbonates; bases and alkalis; bases with ammonium salts; neutralisation, H⁺ + OH⁻ → H₂O. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 3 · Strong and weak acids. Supplement: acids as proton donors and bases as proton acceptors; strong and weak acids, completely and partially dissociated. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 6 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 3 questions, 15 min, lesson closed.
Learning goals
Part 1: Acid, alkali or neutral?
- Describe the colours of litmus, methyl orange and thymolphthalein in acids and alkalis; state that acids contain H+(aq) ions and alkalis OH−(aq) ions; use universal indicator and the pH scale to identify acidic, neutral and alkaline solutions and to compare how acidic or alkaline they are (7.1.2, 7.1.5–7.1.7)
Part 2: What acids and bases do
- Describe the reactions of acids with metals, bases and carbonates, and of bases with ammonium salts, with word equations and their products; describe bases as metal oxides and hydroxides and alkalis as soluble bases; describe neutralisation as H+(aq) + OH−(aq) → H2O(l) (7.1.1, 7.1.3, 7.1.4, 7.1.8)
Part 3: Strong and weak acids
- Supplement: describe acids as proton donors and bases as proton acceptors; describe strong and weak acids as completely and partially dissociated in water, with their equations and state symbols; compare strong and weak acids at the same concentration (7.1.9–7.1.12)
Misconceptions to listen for
- “Litmus turns red in an alkali.” — It is the other way round: an acid turns litmus red, and an alkali turns it blue.
- “Methyl orange is red in an alkali.” — Methyl orange is red in an acid. In an alkali, and in a neutral solution, it is yellow.
- “Row C.” — Thymolphthalein turns blue in an alkali; colourless is its colour in an acid and in a neutral solution.
- “Thymolphthalein turns blue in an acid.” — Thymolphthalein is blue only in an alkali. In an acid, and in a neutral solution, it is colourless.
- “It must be the hydrochloric acid.” — Thymolphthalein is colourless in a neutral solution too, so it cannot prove an acid.
- “It must be the sodium chloride solution.” — Thymolphthalein is colourless in an acid too, so it cannot prove a neutral solution.
- “It must be the sodium hydroxide solution.” — Sodium hydroxide solution, an alkali at pH 13, would turn thymolphthalein blue.
- “Row A.” — A neutral salt solution is full of ions. Neutral means it has no spare H+ or OH− ions.
- “Row B.” — Green is pH 7, neutral, and spare H+ ions would make the solution acidic.
- “Row D.” — Litmus is red only in an acid. In a neutral solution it is purple.
Scope
The characteristic properties of acids and bases (7.1 Core 1 to 8; Supplement 9 to 12): indicator colours, acids and bases with metals, bases, carbonates and ammonium salts, H⁺ and OH⁻ ions, the pH scale and universal indicator, neutralisation, proton donors and acceptors, and strong and weak acids. Acidic, basic and amphoteric oxides are in the module Oxides, preparing salts in Making salts and titration in Titration.