In a solid, the positions of atoms are fixed relative to each other over long time. That is due to the cohesion or “friction” between molecules. This cohesion is provided by metallic, covalent or ionic bonds. Only solids can be pushed on by a force without changing shape, which means that they can be resistant to deformation. Solids also tend to be strong enough to hold their own shape in a container. Solids are generally denser than liquids. Solid becoming a gas is sublimation.
In a liquid, molecules are attracted to other molecules strong enough to keep molecules in contact, but not strong enough to fix a particular structure. The molecules can continually move with respect to each other. This means that liquids can flow smoothly, but not as smoothly as gases. Liquids will tend to take the shape of a container that they are in. Liquids are generally less dense than solids, but denser than gases.
In a gas, the chemical bonds are not strong enough to hold atoms or molecules together, and from this a gas is a collection of independent, unbonded molecules which interact mainly by collision. Gases tend to take the shape of their container, and are less dense than both solids and liquids. Gases have smaller forces of attraction than solids and liquids. Gas becoming a solid is deposition
Plasmas are gases that have so much energy that electrons of an atom cannot stay in orbit around one atomic nucleus. The atomic ions and free electrons mix around like a hot soup.
Phases of matter can be changed by a number of things. This includes pressure and temperature.
When a solid becomes a liquid, it is called melting. When a solid becomes a gas, it is called sublimation. When a liquid becomes a gas, it is called evaporation. When a gas becomes a liquid, it is called condensation. When a liquid becomes a solid, it is called freezing. The freezing point and the melting point are said to be the same, because any increase in temperature will cause it to melt and any drop in temperature will cause it to freeze. This is also the reason that the vaporizing and condensation point are the same.