Vapourphase

Vapourphase is a term used in chemistry and physics to describe the gaseous state of a substance that is normally a liquid or solid at room temperature and pressure. It signifies that the substance has transitioned into a gas through processes such as vaporization or sublimation. Vaporization is the process where a liquid turns into a gas, often by heating. Sublimation is the direct transition from a solid to a gas, bypassing the liquid phase entirely.

The term "vapour" is often used interchangeably with "gas," but there can be subtle distinctions. Vapour specifically refers to a gaseous substance that can be liquefied by increasing pressure alone, without changing the temperature. This implies the substance is below its critical temperature. A gas, on the other hand, is a substance that remains in the gaseous state regardless of pressure changes at a given temperature, if that temperature is above its critical temperature. Therefore, while all vapours are gases, not all gases are considered vapours in this precise thermodynamic definition.

Understanding the vapour phase is crucial in many scientific and industrial applications. It is fundamental to processes like distillation, evaporation, and the operation of engines and refrigeration systems. The behaviour of substances in their vapour phase is governed by gas laws, such as the ideal gas law, which describe the relationships between pressure, volume, temperature, and the number of moles of the gas. Properties like vapour pressure, which is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature, are important characteristics.