Gases

Gases are one of the three classical states of matter, characterized by indefinite shape and volume, the ability to expand to fill any container, high compressibility, and low density compared with liquids and solids. They consist of molecules or atoms in rapid, random motion, and under ordinary conditions intermolecular forces are relatively weak, so gases do not have fixed volumes or shapes.

The kinetic theory explains gas behavior by molecules moving constantly and colliding; these collisions are largely elastic. Temperature relates to the average kinetic energy of particles. This model underpins the gas laws, including Boyle's law, Charles' law, and Avogadro's law, which can be combined into the ideal gas law PV = nRT.

Ideal gases are hypothetical and obey PV = nRT at all conditions. Real gases deviate at high pressure or low temperature due to finite molecular size and intermolecular forces. Corrections are provided by the van der Waals equation or virial expansions to account for non-ideal behavior.

In mixtures, gases follow Dalton's law of partial pressures: the total pressure is the sum of the partial pressures of each component. At fixed temperature and pressure, volumes and mole numbers relate by Avogadro's principle. Diffusion and effusion describe gas movement and flow between regions.

Gases have wide applications in industry, medicine, and science, including industrial synthesis, respiration, anesthesia, and environmental monitoring. The atmosphere is a natural gas mixture, and greenhouse gases influence climate. Safety considerations cover flammability, toxicity, and pressure hazards.