What Is Diffusion?
Particles in gases and liquids are constantly moving in random directions. Diffusion is the overall (net) movement of particles from where they are more concentrated to where they are less concentrated.
This movement continues until particles are spread out evenly — this is called equilibrium. At this point particles are still moving, but there is no overall net movement in either direction.
Concentration gradient is the difference in concentration between two regions. The steeper the gradient (bigger the difference), the faster diffusion occurs.
Everyday Examples
- Smell of perfume spreading across a room — perfume particles diffuse through the air
- Oxygen diffusing from air in the lungs into red blood cells
- Carbon dioxide from respiring cells diffusing into the blood
- Food colouring slowly spreading through still water
What Affects the Rate of Diffusion?
| Factor | Effect on diffusion rate |
|---|---|
| Steeper concentration gradient | Faster — bigger "push" |
| Higher temperature | Faster — particles have more energy and move faster |
| Shorter distance | Faster — less distance to travel |
| Smaller particle size | Faster — lighter particles move faster |
Diffusion in Living Organisms
Many life processes depend on diffusion:
- Gas exchange in lungs: large surface area of alveoli + thin walls + good blood supply make diffusion efficient
- Absorption of digested food in the small intestine: folded walls (villi) maximise surface area
- Osmosis (water movement across membranes) is a special type of diffusion
Real world: Smell from the kitchen
When something is cooking, the aroma spreads from the kitchen to other rooms through diffusion. The volatile molecules travel from high concentration (near the food) to low concentration (rest of the house). Warming up the food increases the speed of this diffusion because particles move faster at higher temperatures.
Real world: Oxygen entering your blood
In the lungs, oxygen concentration is high (just breathed in) while oxygen concentration in the blood is low (used up by body cells). Oxygen diffuses from the alveoli into the blood down this concentration gradient. Carbon dioxide moves the opposite way for the same reason. This is why breathing is essential to life.
Real world: Tea bag in hot water
Drop a tea bag in hot water and watch the brown colour gradually spread through the water even without stirring. The tea molecules diffuse from the concentrated tea leaves into the less concentrated water. Hot water speeds this up because the particles have more kinetic energy and move faster.
Where do you see this in real life? Air fresheners work by diffusion — releasing molecules that spread through a room. Salt diffuses into food when marinating. The scent of flowers diffuses outdoors. Diffusion is happening constantly, invisibly, all around you.