gaseousness

Gaseousness is the quality or condition of matter in the gaseous state, in which a substance has neither a fixed shape nor a fixed volume and expands to fill its container. In gases, molecules move rapidly and are far apart on average, with negligible intermolecular attraction under many conditions.

This state is described by the kinetic theory and the ideal gas law: PV = nRT. For a fixed amount of gas, increasing temperature raises pressure if volume is fixed, or increases volume if pressure is fixed. Gases are highly compressible and have low density compared with liquids and solids. They exhibit rapid diffusion and effusion, and they conduct heat and respond to pressure and temperature changes.

Gaseous versus vapor: a gas is the state of matter for a substance at given conditions; vapor is the gaseous phase of a substance that is liquid or solid at ambient conditions (for example water vapor).

Real gases deviate from ideal behavior at high pressures or low temperatures due to intermolecular forces and molecular size. These deviations are captured by corrections such as the van der Waals equation and by the compressibility factor Z.

Common examples of gaseous matter include air, oxygen, nitrogen, and noble gases. Understanding gaseousness underpins thermodynamics, fluid dynamics, and atmospheric science.