Phase Changes

Up to now, we have considered the behavior of ideal gases. Real gases are like ideal gases at high temperatures. At lower temperatures, however, the interactions between the molecules and their volumes cannot be ignored. The molecules are very close (condensation occurs) and there is a dramatic decrease in volume, as seen in the figure below. The substance changes from a gas to a liquid. When a liquid is cooled to even lower temperatures, it becomes a solid. The volume never reaches zero because of the finite volume of the molecules.

A sketch of volume versus temperature for a real gas at constant pressure. The linear (straight line) part of the graph represents ideal gas behavior—volume and temperature are directly and positively related and the line extrapolates to zero volume at \(–\text{273}\text{.}\text{15}\text{º}\text{C}\)\(–\text{273}\text{.}\text{15}\text{º}\text{C}\), or absolute zero. When the gas becomes a liquid, however, the volume actually decreases precipitously at the liquefaction point. The volume decreases slightly once the substance is solid, but it never becomes zero.

High pressure may also cause a gas to change phase to a liquid. Carbon dioxide, for example, is a gas at room temperature and atmospheric pressure, but becomes a liquid under sufficiently high pressure. If the pressure is reduced, the temperature drops and the liquid carbon dioxide solidifies into a snow-like substance at the temperature \(–\text{78}\text{º}\text{C}\). Solid \({\text{CO}}_{2}\) is called “dry ice.”

Another example of a gas that can be in a liquid phase is liquid nitrogen \(({\text{LN}}_{2})\). \({\text{LN}}_{2}\) is made by liquefaction of atmospheric air (through compression and cooling). It boils at 77 K \((–\text{196}\text{º}\text{C})\) at atmospheric pressure. \({\text{LN}}_{2}\) is useful as a refrigerant and allows for the preservation of blood, sperm, and other biological materials. It is also used to reduce noise in electronic sensors and equipment, and to help cool down their current-carrying wires. In dermatology, \({\text{LN}}_{2}\) is used to freeze and painlessly remove warts and other growths from the skin.