Science lesson plan · 8th grade (Year 9 in England) · 30 minutes
Osmosis and percentage change in mass
Cells, genetics and evolution
CCSS NGSS.HS-LS1-3CCSS NGSS.SEP 5England: GCSE Combined Science: Cell biology (required practical: osmosis)
Objective and success criteria
Students will: Osmosis and percentage change in mass.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Diffusion: how particles spread out
Leads to: Diffusion, osmosis or active transport?
Materials: Microscope images or slides, Cell diagrams, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 4 min
- 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.
- Which change would make diffusion faster? (a higher temperature / a lower temperature / a thicker membrane / a smaller surface area) → a higher temperaturePrompt: Think about how fast the particles move.
- Purple dye spread 8 mm through a block of jelly in 2 hours. What was the mean rate of diffusion, in mm per hour? → 4 mm/hPrompt: rate = distance ÷ time
- Diffusion: which statement is NOT true? (Diffusion does not need energy from respiration. / A steeper concentration gradient gives faster diffusion. / Diffusion moves particles from low to high concentration. / A larger surface area speeds up diffusion.) → Diffusion moves particles from low to high concentration.Prompt: Which direction is the net movement?
Hook · 3 min
- 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.
I do: model it · 7 min
- 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.
We do: guided practice · 6 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- A potato cylinder had a mass of 4 g. After 30 minutes in a sugar solution its mass was 4.8 g. What is the percentage change in mass? (Use a minus sign for a loss.) → 20 %Prompt: percentage change = (final − initial) ÷ initial × 100
- Potato pieces were left in sugar solutions. Percentage change in mass: 0 mol/dm³: +35 % 0.2 mol/dm³: +25 % 0.4 mol/dm³: +15 % 0.6 mol/dm³: +5 % 0.8 mol/dm³: −5 % 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) → 0.6–0.8 mol/dm³Prompt: Where there is no net movement of water, the mass does not change.
You do: independent practice · 7 min
- 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.
Exit ticket · 3 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- A potato cylinder had a mass of 2.4 g. After 30 minutes in a sugar solution its mass was 2.28 g. What is the percentage change in mass? (Use a minus sign for a loss.) → -5 %Prompt: percentage change = (final − initial) ÷ initial × 100
- 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 wilt, because water leaves their cells by osmosis / They grow faster) → They wilt, because water leaves their cells by osmosisPrompt: Compare the concentration inside the stem cells with the salty water.
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Diffusion: how particles spread out.
- Use concrete materials: microscope images or slides.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Move on to: Diffusion, osmosis or active transport?.
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-8.
Free lesson plan from eurekami.com/lesson-plan-builder