• Quick retrieval from earlier learning (Physical and chemical changes). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Is this a physical or a chemical change: cutting paper? (physical change / chemical change) → physical changePrompt: Is a new substance made?
2. A nail left outside turns orange-brown and flaky. What has happened? (Paint fell on it / The nail dissolved / The nail melted / Iron reacted with oxygen and water to make rust) → Iron reacted with oxygen and water to make rustPrompt: Rusting needs both air and water.
3. Physical and chemical changes: which statement is NOT true? (Melting is a chemical change. / Rusting needs both oxygen and water. / In a chemical reaction, atoms are rearranged to make new substances. / Dissolving is a physical change.) → Melting is a chemical change.Prompt: Does each change make a new substance?

• Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
• Share the objective: Acids, alkalis (bases) and the pH scale. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Universal indicator turns red in a solution. What does this tell you?
• Step 1: Red means pH about 0–2: strongly acidic. Green is neutral; purple is strongly alkaline.
• Step 2: So the answer is: It is a strong acid
• Answer: It is a strong acid
• 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. Universal indicator turns red in a solution. What does this tell you? (It is a strong alkali / It is pure water / It is neutral / It is a strong acid) → It is a strong acidPrompt: Red is at the low-pH end of the colour chart.
2. Acids and alkalis: which statement is NOT true? (Alkalis have a pH below 7. / Strong acids and alkalis can be corrosive, so eye protection is needed. / Alkalis are bases that dissolve in water. / Pure water has a pH of 7.) → Alkalis have a pH below 7.Prompt: Lower pH means more acidic.

• 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. Acids and alkalis: which statement is NOT true? (Pure water has a pH of 7. / A pH of 1 is a weak acid. / Alkalis are bases that dissolve in water. / Acids have a pH below 7.) → A pH of 1 is a weak acid.Prompt: Lower pH means more acidic.
2. Which acid is found in your stomach? (sulfuric acid / citric acid / hydrochloric acid / nitric acid) → hydrochloric acidPrompt: It helps kill bacteria in food.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-7.