Aerosols and pressure. Particle matters. Solids liquids and gases.

2. Particle matters

Solids, liquids and gases

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Picture 2.1 The particles in a solid are fixed to their near neighbours. They vibrate around their fixed positions.


		Aerosols rely on solids, liquids and gases and the way they behave. The theory that describes this is the Kinetic Theory of Matter. These are its main points: all matter is made up of particles there are forces between them the particles move about the hotter the substance, the more vigorously its particles move.

	What’s in a solid?
		The particles in a solid are held in a fixed position by bonds. This can be a lattice or it can be a more random arrangement of particles. Either way, they are fixed. The particles vibrate about their fixed position but they are always attached to the same neighbours. As the solid gets hotter, the particles vibrate more vigorously until some of the bonds start to break. At this point, the solid begins to melt. Its temperature stops going up because the energy is being used to break the bonds rather than make the particles vibrate more.

Picture 2.2 The forces at the surface of a liquid hold particles back. However, some escape to form a vapour above the liquid.

	Liquid particles
		Close to the melting point, the arrangement of the particles is quite similar to the solid. However, the particles are not strongly bonded to their neighbours and will begin to move around in the liquid. The bonds between the particles are strong enough to make it difficult for particles to escape from the surface (picture 2.2). So a liquid stays in the bottom of a container.

	Getting the vapours
		However, some of the particles are able to escape from the surface – or evaporate. They form a vapour above the surface of the liquid – the liquid hasn’t boiled so it hasn’t turned into a gas. The vapour is made of a few particles that have escaped from the surface of the liquid. As the liquid is heated, its particles move faster and their average kinetic energy increases. More particles have enough energy to escape from the surface. So the amount of vapour above the liquid goes up. Eventually, at the boiling point of the liquid, all the particles start to escape and the liquid turns into a gas.

Picture 2.3 The particles in a gas are free to fill the space available to them.

	Free as a gas
		Gas particles are free. They have enough energy to stop them from being caught by a bond with another particle. They move around randomly, colliding with each other and with boundary walls. A gas will always expand to fill the container that is holding it. If they bump into a cold wall, they will lose energy and start to bind again with other gas particles. We call this condensing – a gas often forms condensation on a cold surface.

	Liquefied gas propellants
		The propellant in many aerosol cans is a liquefied gas such as butane. Its boiling point is less than room temperature so it is normally a gas. However, it has been put under so much pressure that it has been turned into a liquid inside the aerosol can. Its vapour pushes the mixture of product and propellant out of the can. The propellant will now turn into a gas at room temperature.

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