Diffusion

CORE VS EXTENDED GUIDE

• Core students study only the unlabelled sections.
• Extended students must study everything, including Extended Extended points.
• Extended = Core + Supplement.

Particle Movement and Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. Particles in all fluids [liquids and gases] are in constant random motion, so they spread out and mix without any stirring.

Diffusion occurs because particles have kinetic energy. In a gas, particles move rapidly in all directions. Where particles are more concentrated, more of them move outward than inward, so the substance gradually spreads until the concentration is equal everywhere.

Diffusion in gases is faster than in liquids because gas particles move at much higher speeds and have greater spaces between them, so they encounter less resistance.

A common example is the smell of perfume spreading across a room. The perfume molecules leave the bottle as a gas, collide randomly with air molecules, and gradually reach all parts of the room. No wind or fan is needed — diffusion alone causes the spreading.

MisconceptionStudents sometimes say particles "want" to move from high to low concentration. Particles have no intention — they move randomly in all directions. The net movement toward lower concentration is simply a statistical result of more particles being available to leave the crowded region. Exam cue: always write "net movement" rather than just "movement."

Examiner InsightCIE frequently asks for the definition of diffusion. The three required elements are: (1) net movement, (2) from higher to lower concentration, and (3) particles in a fluid (liquid or gas). Missing any one loses marks. Exam cue: memorise all three parts as a single sentence.

Extended Molecular Mass and Rate of Diffusion

The rate of diffusion of a gas depends on the relative molecular mass (Mᵣ) of its particles. Lighter molecules move faster at a given temperature because the same kinetic energy produces a higher speed in a less massive particle. Therefore, gases with a lower Mᵣ diffuse faster than gases with a higher Mᵣ.

Ammonia (NH₃, Mᵣ = 17) diffuses faster than hydrogen chloride (HCl, Mᵣ = 36.5). When these two gases travel from opposite ends of a glass tube, a white ring of ammonium chloride (NH₄Cl) forms closer to the HCl end, because ammonia covers a greater distance in the same time.

Examiner InsightThe glass-tube experiment with NH₃ and HCl is one of the most frequently examined demonstrations of diffusion. Examiners expect you to state which gas diffuses faster and to justify the position of the white ring. Exam cue: always link speed to lower Mᵣ, then use that to explain the ring's position.