Learning intention: explain net movement by diffusion and apply the idea to gas exchange. This sample has not been mapped to an exact specification or tier.
Guidance for teachers and independent learners
Before starting, recall that particles move randomly. If concentration is unfamiliar, think of it as the number of particles in a given volume. Work through the stages in order or revisit one that needs attention.
Support: use the sentence frame: “There is a net movement from … to … because …”. Stretch: explain how a steeper concentration gradient affects the rate of diffusion, and why particles continue to move at equilibrium.
Teacher prompt: ask learners to distinguish the movement of individual particles from the overall net movement. A common misconception is that all particles travel in the same direction.