A gas is a fluid that expands to occupy its available container and is readily compressed compared with an ordinary liquid. Its pressure reflects microscopic momentum transfer to boundaries. In a sufficiently dilute classical gas, molecular separation is large enough that the ideal gas law can approximate its equilibrium response.
A real gas can depart from that law because intermolecular interactions and finite molecular size become significant. Compressing a gas or changing its temperature therefore tests an equation of state, rather than a universally exact proportionality. A vapor is the gaseous form of a substance under conditions where a condensed phase can be reached by suitable compression or cooling. The same substance can pass continuously between liquid-like and gas-like states around its liquid-gas critical point, so a complete phase description requires a phase diagram as well as the broad word gas.
Gas is one of the four fundamental states of matter, along with solid, liquid, and plasma. In a gaseous state, substances have particles that are widely spaced and move freely, which gives gases the ability to expand to fill the volume of their container. Some key characteristics of gases include: 1. **Indefinite Shape and Volume**: Gases do not have a fixed shape or volume. They take the shape and volume of their container.
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