The physical state of a substance can be changed by changing the pressure. An increase in pressure brings the particles closer and increases the force of attraction between them, that brings about the change.
For example: When high pressure is applied to gas and its temperature is reduced, the gas is converted to a liquid, i.e., the gas is liquified. Thus, by applying pressure and reducing temperature gases can be liquified. Hence, we can say that pressure and temperature determine the state of a substance, whether it will be solid, liquid or gas. The pressure exerted by a gas is measured in the atmosphere. Atmospheric pressure at sea level is taken as one atmosphere which is also normal atmospheric pressure. As we go higher, atmospheric pressure decreases.
