vapeur

Vapeur is the gaseous state of a substance, produced when a liquid or solid changes phase to gas. For the substance water, the common term is vapor d’eau. In scientific contexts, vapeur is used to describe any gas arising from vaporization, whether by boiling or by evaporation at lower temperatures. The term steam is often reserved for water vapor specifically generated by boiling water at or near atmospheric pressure, but in everyday language vapeur can refer to vapors of other substances as well.

Vaporization occurs when heat provides enough energy for molecules to escape the liquid or solid phase. The vapor of a substance exerts a vapor pressure, which reflects its tendency to enter the gas phase at a given temperature. When gas mixes with air, its humidity and dew point determine whether water vapor will condense into liquid droplets. Vapors can be invisible or visible as fog or mist, depending on their temperature and the presence of droplets.

Applications and occurrences: in industry, steam is used for heating, power generation, and process cooling; in chemical and food processing, vaporization aids distillation and sterilization. In natural systems, water vapor is a major component of the atmosphere and a key factor in weather and climate.

Common usage notes: in everyday French, vapeur can also refer to the aerosol emitted by electronic cigarettes (vapoter). This usage differs from combustion smoke and involves tiny droplets and volatile compounds suspended in air.