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Gas is one of the fundamental states of matter, composed of particles with relatively large separations and negligible intermolecular forces under ordinary conditions. Gases have neither fixed shape nor fixed volume and will expand to fill their containers. They are highly compressible and their densities are much lower than liquids or solids.

In kinetic theory, gas molecules move rapidly and collide with each other and container walls, transferring energy. The behavior of gases is described by the gas laws. The ideal gas law PV=nRT relates pressure, volume, temperature, and amount of substance. Boyle's law describes constant temperature; Charles's law describes constant pressure; Avogadro's law relates volume to amount at fixed T and P. Real gases deviate from ideal behavior, especially at high pressure or low temperature; corrections are made by equations such as van der Waals.

Gas mixtures obey Dalton's law of partial pressures; components diffuse and mix, with lighter gases generally diffusing faster than heavier ones (Graham's law). Partial pressures sum to total pressure.

Atmospheric air is a mixture dominated by nitrogen and oxygen, with small amounts of argon, carbon dioxide, neon, helium, methane, water vapor, and others. Gases can be stored as compressed, liquefied, or dissolved forms and are essential in industry, medicine, and research. Common industrial gases include nitrogen, oxygen, carbon dioxide, helium, and argon; natural gas (primarily methane) is used for heating and fuel.

Safety and environmental considerations include flammability, toxicity, and climate impact as certain gases, notably carbon dioxide, methane, and nitrous oxide, act as greenhouse gases. Regulation covers handling, storage, and transport.