• Quick retrieval from earlier learning (Diffusion: how particles spread out). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Someone opens a bottle of perfume in one corner of a still room. Soon it is smelled everywhere. Why? (Air particles stop moving / Perfume particles are pulled by gravity / The perfume becomes a solid / Perfume particles diffuse from high to low concentration) → Perfume particles diffuse from high to low concentrationPrompt: Particles move randomly and spread out.
2. Purple dye spread 12 mm through a block of jelly in 2 hours. What was the mean rate of diffusion, in mm per hour? → 6 mm/hPrompt: rate = distance ÷ time
3. Diffusion: which statement is NOT true? (Diffusion happens in gases and liquids. / A steeper concentration gradient gives faster diffusion. / Diffusion stops completely when the room is cold. / Diffusion does not need energy from respiration.) → Diffusion stops completely when the room is cold.Prompt: Which direction is the net movement?

• Show a microscope image of cells and ask what students think they are looking at.
• Share the objective: Osmosis and percentage change in mass. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A potato cylinder had a mass of 3.2 g. After 30 minutes in a sugar solution its mass was 3.04 g. What is the percentage change in mass? (Use a minus sign for a loss.)
• Step 1: 3.04 − 3.2 = −0.16 g
• Step 2: −0.16 ÷ 3.2 × 100 = −5 %
• Step 3: So the answer is: −5 %
• Answer: -5 %
• 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 potato cylinder had a mass of 2 g. After 30 minutes in a sugar solution its mass was 1.9 g. What is the percentage change in mass? (Use a minus sign for a loss.) → -5 %Prompt: percentage change = (final − initial) ÷ initial × 100
2. Potato pieces were left in sugar solutions. Percentage change in mass: 0 mol/dm³: +15 % 0.2 mol/dm³: +9 % 0.4 mol/dm³: +3 % 0.6 mol/dm³: −3 % 0.8 mol/dm³: −9 % Between which concentrations is the solution that matches the concentration inside the potato cells? (0–0.2 mol/dm³ / 0.2–0.4 mol/dm³ / 0.4–0.6 mol/dm³ / 0.6–0.8 mol/dm³) → 0.4–0.6 mol/dm³Prompt: Where there is no net movement of water, the mass does not change.

• 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. A potato cylinder had a mass of 3 g. After 30 minutes in a sugar solution its mass was 3.3 g. What is the percentage change in mass? (Use a minus sign for a loss.) → 10 %Prompt: percentage change = (final − initial) ÷ initial × 100
2. Cucumber slices sprinkled with salt become limp and watery. Why? (Water leaves the cells by osmosis into the more concentrated salty surface / The cucumber starts to photosynthesise / Salt particles push water out by force / Salt enters and the cells burst) → Water leaves the cells by osmosis into the more concentrated salty surfacePrompt: Water moves from a dilute solution to a more concentrated one.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-8.