A liquid flows under a sustained shear stress while usually resisting a change of volume much more strongly than a gas. It adopts the shape of a container but, when it coexists with a gas and gravity permits, can form a free surface rather than expanding to fill all available space. These are mechanical observations, not a requirement that every liquid be exactly incompressible.
At rest in a uniform gravitational field, a liquid develops the hydrostatic pressure variation needed to support its weight. During flow, viscosity relates rate of deformation to viscous stress for a Newtonian fluid. Releasing a small surface disturbance probes its dynamical response through gravity waves and surface tension. Thus its static pressure, resistance to rapid compression and resistance to shear rate are distinct measurable properties. Liquid and gas can also connect continuously around a liquid-gas critical point, so the broad labels do not imply a first-order boundary along every thermodynamic path.
"Liquid" can refer to several different concepts depending on the context in which it is used. Here are some of the most common meanings: 1. **Physical State of Matter**: In physics and chemistry, "liquid" is one of the three primary states of matter (solid, liquid, and gas). Liquids have a fixed volume but no fixed shape, meaning they take the shape of their container. They are characterized by the ability to flow and conform to the shape of their surroundings.
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