liquide

Liquide is a state of matter characterized by a definite volume but no fixed shape. In a liquid, molecules are less tightly bound than in a solid and can flow past one another, allowing liquids to assume the shape of their container while maintaining their volume.

Liquids are typically incompressible compared with gases, with densities that vary little with pressure but strongly with temperature. Key properties include surface tension, which arises from cohesive forces at the surface; viscosity, a measure of resistance to flow; refractive index; and diffusivity. Some liquids are Newtonian, with constant viscosity, while others are non-Newtonian, whose viscosity changes with stress or shear rate.

Liquids can undergo phase changes: freezing into a solid and vaporization into a gas (evaporation, boiling). They can also exist as intermediate states under high pressure or heat, and at sufficient conditions become supercritical fluids with no distinct liquid-gas boundary. Liquids may mix (miscible) or separate into immiscible layers, forming emulsions or suspensions.

Common examples include water, ethanol, oils, mercury. Liquids play central roles in nature and industry: biology uses cytoplasm and blood plasma as liquids; engineering uses water for cooling, lubrication, or hydraulics; chemistry studies solution behavior and solvent properties.

In physics, liquids are described by continuum mechanics and fluid dynamics, notably the Navier-Stokes equations, which model motion and forces within a liquid. Properties such as density, viscosity, and surface tension are essential for understanding phenomena from droplet formation to capillary rise.

Note: In French and several languages, liquide refers to a liquid; in English, the corresponding term is liquid.