The state of aggregation of matter determines its fundamental appearance, in which, in contrast to the solid and liquid states of aggregation, practically no forces are effective between the molecules. At atmospheric pressure, the density of the gas is reduced by a factor of approximately 1000 in relation to the solid or liquid state, thereby increasing the distance between the molecules accordingly.
Matter changes from the liquid to the gaseous state when the molecular movement in the liquid becomes so large that molecules are broken away from the force field of the neighbouring molecules.
Conversely, by increasing the pressure in a gas, the distance between the molecules is compressed to such an extent that they again come under the influence of mutual forces of attraction. This causes condensation by increasing the pressure at a constant temperature.