It is possible for matter to exist in three states:

The state at which a substance is most balanced at a certain temperature depends on the kinetic energy of the particles at that temperature and how large the attraction between them is.

Remember these three states for your A level chemistry revision

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Solids

The particles of a solid are tightly packed together to form a lattice. In a lattice the particles are ordered in an infinitely repeating arrangement. The particles are attracted to each other in such a way that this ordered arrangement is preserved. In other words, a solid has a fixed shape and volume.

Particles have kinetic energy at any temperature over absolute zero, However, in a solid this kinetic energy is not only not enough to cause the particles to disperse but it causes no significant separation at all. The particles are not able to overcome the potential energy which is caused by their mutual attraction. Instead, the particles are restricted to vibrational and rotational motion as opposed to translational.

As a solid is heated, the kinetic energy of the particles increases causing them to vibrate more. This vibrational motion causes the bonds between the particles to become weak. Some are eventually broken leading to spaces between the particles. The solid is now said to have melted.

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Liquids

Particles in a liquid are also in a basic lattice formation over an amount of 10 to 100 particles. However, over this amount the lattice-like structure breaks down. The particles then cluster together within the additional space. Although the forces between the particles still dominates the properties of liquids, the particles have enough kinetic energy to move into spaces between them. In other words, there is short-range order. A liquid does not have a fixed shape but it does have a fixed volume.

Remember this for A level chemistry revision best results; The particles in a liquid have some translational motion as they can extend to spaces between themselves and other particles. This means that the kinetic energy of these particles is significant even through it is still not strong enough to overcome mutual attraction. If heated strongly enough a liquid will boil.

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Gases

The particles in a gas are constantly in fast and random motion. They behave independently of one another and there is no order at all. The spaces between particles are much larger than the actual particles. As mutual attraction between particles is not significant the properties of a gas are dominated by the kinetic energy of the particles. A gas has no fixed shape or volume.

Important note for A level chemistry revision: due to the fact that kinetic energy is dependent on temperature, all gases behave in a similar fashion at a similar temperature.