• Quick retrieval from earlier learning (Read indicator colours to find pH). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. A solution has a pH of 2. What colour will universal indicator turn? (green / red / orange or yellow / purple) → redPrompt: Universal indicator runs like a rainbow from strong acid to strong alkali.
2. Why is universal indicator more useful than litmus for comparing two acids? (It only works on acids / It is always green / Litmus does not react with acids / It shows a range of colours, so it gives an approximate pH) → It shows a range of colours, so it gives an approximate pHPrompt: Litmus only has two colours.
3. Universal indicator results: lemon juice pH 2, vinegar pH 3, baking soda solution pH 9, oven cleaner pH 13. Which is the most acidic? (lemon juice / baking soda solution / vinegar / oven cleaner) → lemon juicePrompt: The lower the pH, the more acidic.

• Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
• Share the objective: pH and hydrogen ion concentration. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Solution X has pH 5 and solution Y has pH 7. How many times greater is the hydrogen ion concentration in X than in Y?
• Step 1: 10^2 = 100
• Step 2: So the answer is: 100
• Answer: 100
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Solution X has pH 2 and solution Y has pH 4. How many times greater is the hydrogen ion concentration in X than in Y? → 100Prompt: Each drop of 1 pH unit means 10 times more H⁺ ions.
2. A strong acid solution has a hydrogen ion concentration of 0.1 mol/dm³. What is its pH? (pH = −log₁₀[H⁺]) → 1Prompt: Write the concentration as a power of ten.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Solution X has pH 4 and solution Y has pH 6. How many times greater is the hydrogen ion concentration in X than in Y? → 100Prompt: Each drop of 1 pH unit means 10 times more H⁺ ions.
2. What is the difference between a strong acid and a weak acid? (A strong acid fully ionises in water; a weak acid only partly ionises / A strong acid is always more concentrated / A weak acid has no hydrogen / Strong acids are alkalis) → A strong acid fully ionises in water; a weak acid only partly ionisesPrompt: Strength is about ionisation, not concentration.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.