• 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. Carbon dioxide made by respiring cells moves into the blood. Which way is the gradient? (higher in the cells, lower in the blood / there is no carbon dioxide in cells / higher in the blood, lower in the cells / the same on both sides) → higher in the cells, lower in the bloodPrompt: Where is carbon dioxide being made?
2. Agar cubes containing indicator were put into acid. Time for the whole cube to change colour: 1 cm cube 6 min, 2 cm cube 12 min, 3 cm cube 18 min. Which cube's centre was reached fastest by the acid? (the 1 cm cube / the 2 cm cube / the 3 cm cube) → the 1 cm cubePrompt: Fastest means the shortest time.
3. Diffusion: which statement is NOT true? (A steeper concentration gradient gives faster diffusion. / A larger surface area speeds up diffusion. / Diffusion does not need energy from respiration. / Particles stop moving once they are evenly spread.) → Particles stop moving once they are evenly spread.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 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.) → -5 %Prompt: percentage change = (final − initial) ÷ initial × 100
2. Potato pieces were left in sugar solutions. Percentage change in mass: 0 mol/dm³: +25 % 0.2 mol/dm³: +15 % 0.4 mol/dm³: +5 % 0.6 mol/dm³: −5 % 0.8 mol/dm³: −15 % 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. Potato pieces were left in sugar solutions. Percentage change in mass: 0 mol/dm³: +15 % 0.2 mol/dm³: +5 % 0.4 mol/dm³: −5 % 0.6 mol/dm³: −15 % 0.8 mol/dm³: −25 % 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) → 0.2–0.4 mol/dm³Prompt: Where there is no net movement of water, the mass does not change.
2. Fresh flowers are put in a vase of very salty water. What will happen? (They absorb the salt and stay firm / Nothing, because flowers are not alive / They grow faster / They wilt, because water leaves their cells by osmosis) → They wilt, because water leaves their cells by osmosisPrompt: Compare the concentration inside the stem cells with the salty water.

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