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# Diffusion

## Introduction to diffusion

Diffusion is the movement of particles from regions of higher concentration to regions of lower concentration. It primarily involves liquids, gases and dissolved solids.

This phenomenon arises from the random motion of particles within a particular space.

Over an extended period, the particles end up spreading evenly within a space.

You can smell a perfume that was sprayed at the opposite end of a room after some time due to diffusion.

The perfume diffuses from where it was sprayed (regions of higher concentration) to other parts of the room (regions of lower concentration).

Diffusion of two different substances in an enclosed space.

The particles will diffuse until the concentration within a space becomes uniform. Particle movement continues but there will no longer be regions of different concentrations.

## Types of diffusion

Diffusion occurs in various states of matter.

Gas in gas/vacuum diffusion:

A gas released in a container diffuses and fills the container.

When the plate separating the bromine gas and air is removed, the gases diffuse until they are evenly distributed between the two gas jars.

Liquid in liquid diffusion:

When two liquids that are miscible (i.e. they can be mixed) are introduced into a container and left untouched, diffusion occurs until the mixture is uniform.

Solid in liquid diffusion:

Diffusion of cocoa powder (represented by brown circles) in hot water to give a brown mixture.

The particles of a solid that dissolve in a liquid similarly spread throughout the volume of the liquid.

## Effect of temperature and molecular mass on the rate of diffusion

The rate of diffusion is affected by molecular mass and temperature.

These two factors affect the kinetic energy of a particle (the energy a particle possesses when it moves).

The rate of diffusion of a substance is inversely proportional to its molecular mass at constant temperature. This means that the rate of diffusion increases as the molecular mass decreases.

Chlorine gas $(\text{71 g/mol})$ diffuses more slowly than hydrogen gas $(\text{2 g/mol})$ when both molecules have the same kinetic energy.

The rate of diffusion of a substance is directly proportional to the temperature. This means that the rate of diffusion increases as the temperature increases.

This is because the molecules of a substance at a higher temperature have greater kinetic energy (and higher velocity). They therefore diffuse at a greater rate.

Instant coffee diffuses more quickly in hot water than in cold water.