States of Matter and Phase Changes
Solids
In the solid state, the particles are touching, and the only motion allowed to them is vibration. The particles may be arranged regularly (in which case, the solid is crystalline), or at random (giving waxy solids like candles or some forms of polyethylene, for example).
The particles are held in the solid by forces which depend on the actual substance – ionic bonds, covalent bonds, metallic bonds, hydrogen bonds, or van der Waals attractions.
Liquids
In a liquid, the particles are mainly touching, but some gaps have appeared in the structure. These gaps allow the particles to move, and so the particles are arranged randomly.
The forces that held the solid particles together are also present in the liquid (unless melting has broken up a substance consisting only of covalent bonds – a giant covalent structure). However, the particles in the liquid have enough energy to prevent the forces holding them in a fixed arrangement.
For most liquids, the density of the liquid is slightly less than that of the solid, but there isn’t much difference. That means that the particles in the liquid are almost as close together as they are in a solid. If you draw diagrams of liquids, make sure that most of the particles are touching, but at random, with a few gaps.
Gases
In a gas, the particles are entirely free to move. At ordinary pressures, the distance between individual particles is of the order of ten times the diameter of the particles. At that distance, any attractions between the particles are negligible at ordinary temperatures and pressures.
Boiling and Condensing
If more heat energy is supplied, the particles eventually move fast enough to break all the attractions between them, and the liquid boils. The heat energy required to convert 1 mole of liquid into a gas at its boiling point is called the enthalpy of vaporization.
If the gas is cooled, at some temperature the gas particles will slow down enough for the attractions to become effective enough to condense it back into a liquid. Again, as those forces are reestablished, heat energy is released.
Evaporation in a Closed Container
Now imagine what happens if the liquid is in a closed container. Common sense tells you that water in a sealed bottle doesn’t seem to evaporate – or at least, it doesn’t disappear over time.
But there is constant evaporation from the surface. Particles continue to break away from the surface of the liquid – but this time they are trapped in the space above the liquid.
As the gaseous particles bounce around, some of them will hit the surface of the liquid again, and be trapped there. There will rapidly be an equilibrium set up in which the number of particles leaving the surface is exactly balanced by the number rejoining it.
In this equilibrium, there will be a fixed number of gaseous particles in the space above the liquid.
When these particles hit the walls of the container, they exert a pressure. This pressure is called the saturated vapor pressure (also known as saturation vapor pressure) of the liquid.