Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, due to the random motion of the particles.

How diffusion works

All particles in liquids and gases are in constant random motion. In a region of high concentration, there are more particles. Because their motion is random, more particles move out of the high-concentration region than into it. Over time, the particles spread out until the concentration is uniform throughout.

Evidence for particle theory

Diffusion provides evidence that particles are in constant random motion:

• A drop of ink in water spreads out on its own, without stirring.
• Bromine gas released in a jar of air gradually fills the entire jar.
• The smell of perfume spreads across a room.

These observations can only be explained if the molecules are moving and colliding randomly.

Factors that affect the rate of diffusion

Temperature: higher temperature means particles have more kinetic energy and move faster. Diffusion is faster at higher temperatures.

Mass of particles: lighter particles diffuse faster than heavier ones at the same temperature. Hydrogen gas (light) diffuses faster than bromine gas (heavy).

Concentration difference: a larger concentration difference produces faster diffusion because there is a greater net movement of particles.

State of matter: diffusion is fastest in gases (particles are far apart and move freely) and slowest in liquids (particles are closer and experience more collisions). Diffusion does not occur in solids because the particles are fixed in position.

Diffusion vs convection

Diffusion is driven by random particle motion (concentration difference). Convection is driven by density differences caused by heating (bulk fluid movement). They are different processes, though both can transfer particles or energy through a fluid.