gasphase

Gas phase refers to the state of matter in which a substance or mixture is predominantly in gaseous form. In this phase, substances lack a fixed shape and volume, instead expanding to fill the available space and being highly compressible. Gas molecules move with rapid, random motion and interact mainly through short-range collisions; attractive and repulsive forces are often small compared with thermal energy at moderate temperatures.

Under many conditions, gases follow the ideal gas law PV = nRT, and can be described by kinetic theory: particles have negligible size relative to distances between them, their collisions are elastic, and the macroscopic properties arise from average molecular motion. Real gases deviate at high pressures or low temperatures due to intermolecular forces and finite molecular size.

Gas-phase chemistry encompasses reactions that occur in the gas phase, including atmospheric reactions, combustion, and synthesis at low pressures. Reaction rates depend on concentrations, temperature, pressure, and energy transfer in collisions. Concepts such as collision theory, transition state theory, and Arrhenius kinetics model these processes. Diffusion and mixing in the gas phase are rapid; diffusion often governs rates in small volumes or systems with limited mixing.

Techniques and applications include gas-phase spectroscopy, which examines vibrational and rotational transitions of molecules in the gas phase; gas chromatography uses a carrier gas as the mobile phase; mass spectrometry often introduces analytes in the gas phase after ionization. Gas-phase methods are central to analytical chemistry, atmospheric science, and many industrial processes.