Liquides

Liquids are a state of matter defined by a definite volume but no fixed shape; they flow to fill the lower part of their container and take the container's shape. The molecules in a liquid are less tightly bound than in a solid, allowing diffusion and deformation under stress. Liquids are almost incompressible under ordinary pressures, and their movement is described by hydrodynamics.

Important properties include density, viscosity, surface tension, boiling point, freezing point, vapor pressure, and refractive index. Density measures mass per unit volume; viscosity measures resistance to gradual deformation; surface tension arises from cohesive forces at the interface with air or another phase. Boiling point and freezing point denote phase transitions with temperature; vapor pressure reflects evaporation tendency at a given temperature.

Liquids can be classified by composition as pure liquids or mixtures (solutions or suspensions). They can also be categorized by polarity (polar vs nonpolar), volatility (volatile vs nonvolatile), and chemical structure (organic vs inorganic). Interfacial phenomena such as capillarity and wetting describe how liquids interact with solids.

In nature and technology, liquids serve as solvents, reactants, heat-transfer media, lubricants, and coolants. Water is the most ubiquitous example; organic solvents like ethanol, acetone, and hydrocarbons are common in laboratories and industry. Many liquids are miscible or immiscible in one another, leading to layered systems or single-phase solutions.

Measuring liquids involves standard units of density (g/cm3 or kg/L), dynamic viscosity (Pascal-second), surface tension (N/m), and boiling/freezing points. Understanding liquid behavior under temperature and pressure changes is central to chemistry, physics, engineering, and materials science.