Liquids

Liquids are one of the fundamental states of matter. They have a definite volume but no fixed shape, flowing to conform to the shape of their container. Liquids are nearly incompressible under ordinary pressures and possess densities typically higher than those of gases and lower than most solids. They exhibit cohesion, the attractive forces between like molecules, and adhesion to surfaces, which governs wetting and capillary action.

Key physical properties include viscosity, which measures resistance to flow; surface tension, arising from cohesive forces at the liquid’s surface; and vapor pressure, the tendency to evaporate. Temperature and pressure strongly influence these properties: increasing temperature generally lowers viscosity and raises vapor pressure, and liquids undergo phase transitions at characteristic melting and boiling points. Liquids can exist as pure substances or as mixtures; some liquids mix completely (miscible) to form solutions, while others form separate immiscible layers.

Molecular interactions such as hydrogen bonding and polarity determine the behavior of a liquid. These interactions influence how liquids spread on surfaces, climb thin tubes, or interact with other substances. In practical terms, viscosity and wetting properties affect lubrication, coating, and fluid transport. Rheology distinguishes Newtonian liquids, with constant viscosity, from non-Newtonian liquids, whose viscosity changes with shear rate.

Natural and technological contexts include water in the environment, biological fluids such as blood, and industrial solvents and fuels. The phase behavior of liquids is described by pressure–temperature diagrams showing regions of solid, liquid, and gas stability.