gaseous

Gaseous is the adjective used to describe matter in the gas state. A gas expands to fill its container, has no fixed shape or volume, and can mix with other gases. The gaseous state is one of the fundamental states of matter, alongside solids and liquids.

In the gaseous state, molecules are widely spaced, move rapidly, and exhibit considerable compressibility. Gases diffuse readily and mix by molecular motion. The kinetic molecular theory explains these properties: particles are in constant, random motion with mostly empty space and only weak intermolecular forces.

Gas behavior is described by gas laws. The ideal gas law, PV = nRT, relates pressure, volume, temperature, and amount of gas under moderate conditions. Real gases deviate at high pressure or low temperature due to intermolecular interactions and finite molecular size, which can be described by corrected equations such as the van der Waals model.

Common gaseous substances include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), methane (CH4), hydrogen (H2), and the noble gases like helium and argon. Many elements are gaseous at room temperature, while others become gases only at elevated temperatures.

Gaseous can describe the gas phase of a substance that is typically liquid or solid at room temperature. The term vapor is often used for the gas phase during a phase transition, whereas gaseous denotes the state more generally.

Applications span respiration, combustion, industrial gas production, and laboratory techniques that rely on gases. In the environment, some gases act as greenhouse gases affecting climate, while others are managed to reduce emissions.