Lesson Plan

Resources → Subject Notes → Chemistry → Notes
Lesson Plan
Grade: Date: 17/01/2026
Subject: Chemistry
Lesson Topic: Describe and explain the effect of relative molecular mass on the rate of diffusion of gases
Learning Objective/s:
  • Describe diffusion and state Graham’s law of diffusion.
  • Explain how relative molecular mass influences the rate of diffusion.
  • Apply Graham’s law to calculate and compare diffusion rates of different gases.
  • Analyse the effect of temperature, pressure and medium on diffusion.
  • Solve typical exam‑style questions involving diffusion rates.
Materials Needed:
  • Projector and screen
  • Whiteboard and markers
  • Printed handout with gas table and formula
  • Worksheet with practice questions
  • Scientific calculators
  • Interactive diffusion simulation (e.g., PhET)
 Introduction:
Imagine opening a bottle of perfume and watching the scent fill the room in seconds. Students already know about kinetic theory and concentration gradients, so we’ll link that knowledge to diffusion. Success will be measured by students correctly stating Graham’s law and predicting which gas diffuses faster in a given scenario.
Lesson Structure:
  1. Do‑now (5') – Quick written response: “What is diffusion and why does it occur?”
  2. Mini‑lecture (10') – Introduce Graham’s law, derive the formula, and show a diagram of light vs. heavy gases diffusing.
  3. Guided practice (12') – Using the provided gas table, calculate diffusion rate ratios for three pairs of gases.
  4. Interactive simulation (8') – Students manipulate temperature and molecular mass in a PhET model and record observations.
  5. Independent worksheet (10') – Solve an exam‑style question comparing Cl₂ and NH₃ diffusion rates.
  6. Check for understanding (5') – Exit ticket: one sentence explaining why lighter gases diffuse faster.
Conclusion:
We will recap Graham’s law, the inverse √M relationship, and the influence of other factors. Students will hand in their exit tickets, and the teacher will summarise common misconceptions. Homework: complete a worksheet with three additional gas‑pair calculations and a short reflection on how temperature could modify the results.