The four phases of matter are the solid, liquid, gaseous, and plasma states. Solid phases are found at lower temperatures; as the amount of internal energy increases, the material changes from the solid to the liquid and then to the gaseous phase and finally, under extreme conditions, to the plasma state. Water, for example, changes from ice, its solid state, to liquid water, and finally to steam, its gaseous state. The temperatures at which the phase changes occur depend on the properties of the material.

In the solid phase the atoms or molecules are held in rigid positions by the chemical bonds between them. They can vibrate, but not change positions.

When the temperature is at the freezing point the solid melts into a liquid. In the liquid phase molecules, or small groups of molecules can move easily past one another. In most liquids the spacings between the molecules is slightly larger than in solids, giving them a lower density. In water, however, the spaces are larger in ice than in liquid, meaning that ice has a lower density than water, so it floats.

When the temperature reaches the boiling point the liquid becomes a gas. In the gaseous phase the atoms or molecules have essentially no forces between them, so they are free to move independently. They are about 10 times further apart than in a liquid or solid, meaning that the density of a typical gas is 1/1,000 that of the solid.

To enter the plasma state one or more electrons must be removed from the atom. Plasmas consist of electrically charged particles. Plasmas are found in fluorescent lamps, in some television displays, in the upper atmosphere, in the sun and other stars, and in interstellar space.