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

1. Is this exothermic or endothermic: neutralising an acid with an alkali? (exothermic / endothermic) → exothermicPrompt: Does it give out heat or take heat in?
2. The temperature of a reaction mixture changes from 22 °C to 7 °C. What is the temperature change? (Use a minus sign for a fall.) → -15 °CPrompt: Change = final temperature − starting temperature.
3. In an exothermic reaction, the energy of the products is… (higher than the energy of the reactants / lower than the energy of the reactants / exactly the same as the reactants / zero) → lower than the energy of the reactantsPrompt: Energy has been given out to the surroundings.

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

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A reaction produces 150 cm³ of gas in 50 seconds. What is the mean rate of reaction, in cm³/s?
• Step 1: 150 ÷ 50 = 3
• Step 2: So the answer is: 3 cm³/s
• Answer: 3 cm³/s
• 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. A reaction produces 20 cm³ of gas in 20 seconds. What is the mean rate of reaction, in cm³/s? → 1 cm³/sPrompt: mean rate = amount of product ÷ time
2. Why does increasing the temperature speed up a reaction? (It adds more particles / Particles get bigger / It lowers the activation energy / Particles move faster, collide more often and more collisions have enough energy) → Particles move faster, collide more often and more collisions have enough energyPrompt: Think about collision theory.

• 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 reaction produces 50 cm³ of gas in 20 seconds. What is the mean rate of reaction, in cm³/s? → 2.5 cm³/sPrompt: mean rate = amount of product ÷ time
2. Why does a more concentrated acid react faster? (It has a lower pH so no collisions are needed / It is hotter / There are more acid particles in the same volume, so collisions are more frequent / The particles are heavier) → There are more acid particles in the same volume, so collisions are more frequentPrompt: Concentration is how crowded the particles are.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.