Diffusion
Diffusion
- Diffusion is the spreading of particles from high concentration to low concentration.
- It happens because particles are always moving in random directions.
- It explains smelling food across a room.
- Diffusion happens in gases and liquids, but not solids (their particles can't move from place to place).
Practice
Diffusion is the movement of particles from a region of:
Particles spread out from where they are crowded (high concentration) to where they are spread out (low).
Practice
Diffusion happens in gases and liquids, but not in solids.
Solid particles only vibrate in place, so they cannot move from place to place and diffuse.
Lighter gases diffuse faster
- At the same temperature, lighter particles move faster.
- So a gas with a smaller relative molecular mass ($M_r$) diffuses faster.
- Classic test: ammonia ($M_r = 17$) and hydrogen chloride ($M_r = 36.5$) diffuse along a tube and form a white ring of ammonium chloride. The lighter ammonia travels further, so the ring forms nearer the HCl end.
Practice
Which gas diffuses faster at the same temperature?
Lighter particles (smaller Mr) move faster, so they diffuse faster — ammonia beats hydrogen chloride.
You've got it
Key idea
- diffusion = spreading from high to low concentration, in gases and liquids (not solids)
- lighter gases (smaller $M_r$) diffuse faster
- ammonia ($M_r=17$) beats HCl ($M_r=36.5$), so the white ring forms nearer the HCl end