In a solid the particles are ordered, this means they have a regular arrangement - just like the diagram on the right. They're held in a fixed position - i.e. they can't move. All they can do is vibrate around a fixed position. This is because there are strong attractive forces holding them in place.
The strong forces holding the particles in place gives solids a fixed shape/volume. It also means that the particles cannot flow over each other and that the solid form of a substance is normally the most dense.
In a liquid the particles can move. The particles in a liquid are closely packed together and are in contact with each other but they lack the order of a solid. The particles are disordered and move at random. The forces holding the particles together in a liquid are strong but not strong enough to prevent them from moving.
The particles in a liquid can move, this means that they can flow and change their shape to match whatever container holds them. They have a fixed volume and are slightly less dense than a solid.
In a gas the particles are separated by large distances and move randomly. In this state the particles have enough energy to overcome the forces holding them in place and move freely.
The large distance between particles gives gases a low density The large distance between particles also means that they can be compressed (squashed) into a smaller volume. Or in more scientific language their volume changes with pressure. As the particles can move freely gases can flow and they fill whatever container they are placed in.
The table below is a summary of the properties of solids, liquids and gases.
|Liquid||Matches Container Shape||Fixed||Slightly lower than the solid||Move randomly|
|Gas||Fills Container||Varies||Very low||Move freely|