• 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? (Perfume particles diffuse from high to low concentration / Perfume particles are pulled by gravity / Air particles stop moving / The perfume becomes a solid) → Perfume particles diffuse from high to low concentrationPrompt: Particles move randomly and spread out.
2. Purple dye spread 6 mm through a block of jelly in 3 hours. What was the mean rate of diffusion, in mm per hour? → 2 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. / Particles stop moving once they are evenly spread. / A larger surface area speeds up diffusion.) → 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: Surface area to volume ratio and exchange. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A cube-shaped cell model has sides of 3 cm. What is its surface area to volume ratio, written as ? : 1?
• Step 1: surface area = 6 × 3 × 3 = 54 cm²
• Step 2: volume = 3³ = 27 cm³
• Step 3: 54 ÷ 27 = 2
• Step 4: So the answer is: 2
• Answer: 2
• 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 cube-shaped cell model has sides of 2 cm. What is its surface area to volume ratio, written as ? : 1? → 3Prompt: Surface area of a cube = 6 × side × side.
2. Why does a large multicellular animal need lungs and a heart, but an amoeba does not? (Amoebae are not alive / Large animals do not use oxygen / Its small SA:V ratio and long distances mean diffusion alone is too slow / Diffusion is faster in big animals) → Its small SA:V ratio and long distances mean diffusion alone is too slowPrompt: How far is the middle of your body from your skin?

• 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. Why does a large multicellular animal need lungs and a heart, but an amoeba does not? (Diffusion is faster in big animals / Large animals do not use oxygen / Amoebae are not alive / Its small SA:V ratio and long distances mean diffusion alone is too slow) → Its small SA:V ratio and long distances mean diffusion alone is too slowPrompt: How far is the middle of your body from your skin?
2. A cube-shaped cell model has sides of 3 cm. What is its surface area to volume ratio, written as ? : 1? → 2Prompt: Surface area of a cube = 6 × side × side.

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