Cambridge IGCSE Chemistry · Syllabus 0620 · States of Matter
Diffusion
What is Diffusion?
The net movement of particles from a region where they are at a higher concentration to a region where they are at a lower concentration, brought about by the random motion of the particles. Individual particles move in all directions and change direction whenever they collide; because there are more particles on one side to begin with, more happen to cross away from that side than towards it, so over time the substance becomes evenly spread. Diffusion needs no stirring and no external push, it happens faster at higher temperatures, and it is fastest in gases because their particles are far apart and move rapidly.
This definition is part of the States of Matter chapter in Cambridge IGCSE Chemistry.
Examiner tips on Diffusion
- This is the exception in Topic 1, so read the statement carefully. Everywhere else in this chapter, Core describes and Supplement explains. Statement 1.2.1 asks every candidate to describe and explain diffusion in terms of kinetic particle theory, so the whole particle mechanism in this lesson — random motion, net movement, the counting argument — is Core material and can be asked on the Core papers. Only the effect of relative molecular mass on the rate of diffusion is Supplement, and that is the separate statement 1.2.2 in Lesson G.
- Context, not an examinable claim The honest limits of these two examples. In a real room, a smell usually reaches you faster than diffusion alone would carry it, because warm air rises and cool air sinks and those currents carry the particles along. In a beaker, any vibration or difference in temperature sets up currents too. Diffusion is the part that would still happen in perfectly still surroundings — and it is that part, not the currents, that Topic 1.2 is about.