What is Diffusion?
The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion.
Diffusion is one of the most important processes in both chemistry and biology. It explains how particles — whether they are molecules in a gas or solute particles in a liquid — spread out over time without any external force pushing them.
The key idea is that particles are always moving randomly. When there are more particles in one place than another, the randomness of their motion naturally causes a net movement from the crowded region to the less crowded region. Eventually, the particles spread out evenly — we call this reaching equilibrium.
Imagine a crowded school corridor at lunchtime. Students (particles) move randomly. If one end of the corridor is packed and the other is empty, more students end up drifting toward the empty end — not because they planned to, but because there is simply more room. This is exactly how diffusion works!
Diffusion does not require energy input — it is a passive process driven entirely by random particle motion and the concentration gradient.
Concentration Gradients
The difference in concentration of a substance between two regions. Particles naturally move down the concentration gradient (from high to low concentration).
The amount of a substance dissolved or present in a given volume, often measured in mol/dm³ or g/dm³.
Think of the concentration gradient as a slope — particles naturally "roll" down this slope from high concentration to low concentration, much like water flows downhill.
- The steeper the gradient (the bigger the difference in concentration), the faster diffusion occurs.
- When concentrations are equal on both sides, equilibrium is reached. Particles still move, but there is no net movement in either direction.
A common misconception is that diffusion stops at equilibrium. It does not! Particles continue to move randomly, but because the concentration is equal everywhere, there is no overall (net) movement in any one direction.
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