Matter exists in four different forms - solid, liquid, gas, and plasma. These are the four states of matter. You can recognize each state because they have different properties. A property of a material describes how it behaves. This behavior is not observable at the chemical level due to microscopic sized particles. We can observe how many H2O particles behave when we open a faucet, releasing water, which is a lot of H2O particles in their liquid state. Of course we can observe other states of H2O particle groupings, at low temperatures they freeze, demonstrating a solid property. At high temperatures they form a gas called steam.
The particles that are in a substance (material) stay the same whatever state the substance is in. The only thing that changes is the arrangement of the particles and the energy they have.
Whichever state the substance is in depends on how strongly the particles stick together. And how well the particles stick together depends on:
the material
the temperature
the pressure.
This is particle theory, and it explains all the different properties of solids, liquids and gases.
Particles are still touching and held by very strong forces of attraction.
They are free to move past each other.
Because the particles can move, liquids don't have a definite shape, and they can flow.
Because the particles are still packed close together, liquids can't easily be compressed and keep the same volume.
Because there are lots of particles in a small volume, liquids have almost the same density as when they are solid. Some, like water are actually more dense than their solid, but most are very slightly less dense.
Particles are far apart from each other because there are very weak forces of attraction between them.
They move fast in all directions.
Because of this, gases don't have a definite shape or volume and fill any container.
Because there is lots of free space between particles, gases can easily be compressed.
Because there aren't many particles in a large volume, gases have very low densities typically 1000 times less than in their condensed state as solid or liquid at atmospheric temperature and pressure.
