Particles are always moving
Particles move randomly and collide. Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
Diffusion & osmosis
Follow the movement of particles and water across a membrane.
Class notes
Open PDF →Across a membrane
A selectively permeable membrane allows some substances to pass more readily than others. Whether a substance crosses depends on the substance and the membrane.
Osmosis is about water
Osmosis is the net movement of water across a selectively permeable membrane. A difference in solute concentration can affect the direction of net water movement.
Explore the idea
Particles move randomly in both directions; the arrow shows the net direction. Whether particles cross depends on the membrane.
01Diffusion
Particles move randomly; overall movement tends from higher to lower concentration.
02Selectively permeable membrane
Some substances cross more readily than others, depending on the membrane and the substance.
03Osmosis
Osmosis is the net movement of water across a selectively permeable membrane.
Explore this idea further
Watch: How does the cell membrane work?
Follow the movement of particles and water across a membrane.
Open on YouTube →Why might a plant cell become firm in a dilute surrounding solution?
Reveal the reasoning
Water can move into the cell by osmosis; the cell wall resists expansion and pressure develops.
Try it yourself.
A membrane lets water pass but not a dissolved solute. Compare net water movement when one side has more solute.
Check your thinking
Under these conditions, net water movement is toward the side with more solute until the driving forces balance. Water molecules still move in both directions.
Common question: Do particles stop moving at equilibrium?
No. Random motion continues, but there is no net movement across the system at dynamic equilibrium.
Take it further.
If a smell spreads across a room, why is diffusion a useful model?
See a possible explanation
Gas particles move randomly and spread from a region of higher concentration toward areas of lower concentration.