Chapter 1: States of matter — Part 2

01 Definition of diffusion

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.

02 Kinetic particle theory and diffusion

Whether diffusion occurs depends on how freely particles can move, as explained by kinetic particle theory:

Solids

Diffusion does not occur. Particles are in a fixed arrangement and can only vibrate in place.

Liquids and gases

Diffusion does occur. Particles are able to move, collide, and spread apart, allowing them to move from regions of higher concentration to lower concentration.

03 Effect of relative molecular mass on diffusion

Relative molecular mass (M_r) is the sum of the atomic masses of all atoms in a molecule. For example, CO₂ has M_r = 44 (12 + 16 + 16).

Heavier particles (larger M_r)

Diffuse at a slower rate because they move more slowly.

Lighter particles (smaller M_r)

Diffuse at a faster rate because they move more quickly.