Gas

A state of matter whose particles move freely, causing it to expand to fill its container and respond readily to compression.

Kinetic theory of gases: A model that explains gas properties through the motion and collisions of microscopic particles. It connects the motion of gas particles to pressure, temperature, and volume.
Matter: Physical substance that has mass and occupies space. Gas is one of the common physical states in which matter occurs.
Liquid: A state of matter with nearly fixed volume that flows and conforms to its container. Unlike a gas, a liquid does not expand to fill all available space.
Compressed air: Air stored above atmospheric pressure for use as an energy carrier or working fluid. Its storage and release depend directly on gas compressibility.
Van der Waals equation: An equation of state that approximates real-gas behavior by accounting for molecular attraction and finite particle volume. It corrects two assumptions that make the ideal gas law fail for real gases.
Gas pressure: The force per unit area exerted by gas particles colliding with a surface. Molecular collisions with container walls create the pressure characteristic of gases.
Particle: A small constituent unit of matter, such as an atom, molecule, or ion. Gas behavior arises from the motion and interactions of its constituent particles.
Solid: A state of matter that maintains a defined shape and volume under ordinary conditions. Its particles remain in fixed arrangements rather than moving freely through a container.
Pneumatics: The use of pressurized gas to transmit force and power mechanical systems. Pneumatic devices convert gas pressure into motion.
Critical point: The temperature and pressure at which a substance's liquid and gas phases become indistinguishable. Near this point, gas-like and liquid-like behavior merge into a single fluid phase.

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