In all 4 states, even a solid, the particles are always moving. This constant movement is called the Kinetic Theory of matter.
The amount of kinetic energy (movement) the particles have, will reflect on their state (solid, liquid or gas).
The more energy we give to these particles, the more states they can become.
How can we use this theory?
We know that all particles have some form of mass. The mass is the amount of matter in a substance. We can measure this in kg.
A particular volume of solid or liquid has a greater mass than the same volume of gas.
For example, a container of liquid nitrogen is heavier, than the same sized container of nitrogen gas.
What you will also notice, is how some metals are heavier than others. A piece of lead has a greater mass than a piece of aluminium.
Both are solid and have tightly packed atoms. The difference is the mass of the atoms. A lead atom, has a higher atomic mass than an aluminium atom.
Have you ever noticed metal expanding in heat? Probably not because the change is so small, but when we heat something it expands. This links back to our kinetic theory.
Solids are made of atoms which are very close together and vibrate a little bit. When we heat them and give them energy they move a bit more. They move faster, taking up more space and pushing the others out. This makes the metal expand. This is the same reason why a thermometer rises when introduced to heat.