• 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 8. What colour will universal indicator turn? (blue / red / orange or yellow / green) → bluePrompt: Universal indicator runs like a rainbow from strong acid to strong alkali.
2. Blue litmus paper is dipped into lemon juice. What happens? (It turns purple / It turns red / It stays blue / It turns green) → It turns redPrompt: Lemon juice is acidic.
3. Universal indicator results: baking soda solution pH 9, oven cleaner pH 13, vinegar pH 3, pure water pH 7. Which is the most alkaline? (vinegar / baking soda solution / pure water / oven cleaner) → oven cleanerPrompt: 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 3. How many times greater is the hydrogen ion concentration in X than in Y? → 10Prompt: Each drop of 1 pH unit means 10 times more H⁺ ions.
2. A strong acid solution has a hydrogen ion concentration of 0.001 mol/dm³. What is its pH? (pH = −log₁₀[H⁺]) → 3Prompt: 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 6 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. A strong acid solution has a hydrogen ion concentration of 0.000001 mol/dm³. What is its pH? (pH = −log₁₀[H⁺]) → 6Prompt: Write the concentration as a power of ten.
2. Ocean water's pH has fallen from about 8.2 to about 8.1 since the 1800s. Roughly how has the H⁺ concentration changed? (it has not changed / it has fallen by half / it has increased 10 times / it has increased by about 25–30 %) → it has increased by about 25–30 %Prompt: A drop of 1 pH unit is ×10; a drop of 0.1 is 10^0.1 ≈ 1.26.

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