If someone is cooking in the kitchen, the smell travels around the house to other rooms. This is because of diffusion, the movement of particles from an area of high concentration to an area of low concentration. For diffusion to work, the particles must be able to move around. This means that diffusion does not happen in solids – the particles in a solid can only vibrate and cannot move from place to place.
Diffusion is driven by differences in concentration. When chemical substances such as perfume are let loose in a room, their particles mix with the particles of air. The particles of smelly gas are free to move quickly in all directions. They eventually spread through the whole room from an area of high concentration to an area of low concentration. This continues until the concentration of the particles of smelly gas is the same throughout the room. Remember that the particles still move, even when the smell is evenly spread.
You do not have to mix the gases by waving your arms around - they mix on their own. Diffusion in gases is quick because the particles in a gas move quickly. It happens even faster in hot gases because the particles of gas move faster.
Gas particles move very quickly (air particles move at 500 m/s on average at room temperature). However, a smell does not travel this fast. This is because its particles collide with each other and with particles of air very frequently. They change direction randomly when they collide, so it takes much longer to travel from one place to another. Their random motion because of collisions is called Brownian motion.
Diffusion can also happen in liquids. This is because the particles in liquids can move around each other, which means that eventually they are evenly mixed.
For example, potassium manganate(VII) is a purple solid. If you put a crystal of it into a jar of water, the purple colour spreads slowly through the water. This is by diffusion. The slideshow describes what happens.
Diffusion in liquids is slower than diffusion in gases because the particles in a liquid move more slowly. It happens faster if the temperature is increased.