A liquid is one of three classical states of matter, the other two being solid and gas.

The density of a liquid is typically of the same order as of the corresponding solid, and much higher than in the gas. Therefore, liquid and solid matter are jointly designated as condensed matter. On the other hand, a liquid shares with a gas the ability to flow. Therefore, liquids and gases are also called fluids.

Examples and classes of liquids

Only two elements are liquid at room temperature and pressure: mercury and bromine. Four more elements have melting points slightly above room temperature: francium, caesium, gallium and rubidium.

Pure substances that are liquid under normal conditions include water, ethanol and many other organic solvents. Liquid water is of primordial importance in chemistry and biology; it is believed to be a necessity for the existence of life.

Important everyday liquids include aquous solutions like household bleach, other solutions (homogeneous mixtures, multiphasic liquids) like mineral oil and gasoline, emulsions like vinaigrette or mayonnaise, suspensions like milk and blood, and colloids like paint.
Liquid crystals, used in LCD displays, cannot be classified within the classical three states of matter; they possess solid-like and liquid-like properties. The same holds for biological membranes.

Mechanics of liquids

Quantities of liquids are commonly measured in units of volume. These include the SI unit cubic metre (m³) and its divisions, in particular the cubic decimetre, more commonly called litre (dm³=l), and the cubic centimetre, also called millilitre (cm³=ccm=ml).

A liquid's shape is determined by the container it fills. That is to say, liquid particles (normally molecules or clusters of molecules) are free to move about the volume, but they form a discrete surface that may not necessarily be the same as the vessel. The same cannot be said about a gas. It can also be considered a fluid, but it must conform to the shape of the container entirely.

The volume of a quantity of liquid is fixed by its temperature and pressure. Unless this volume exactly matches the volume of the container, (one or more) surfaces are observed. Liquids in a gravitational field, like all fluids, exert pressure on the sides of a container as well as on anything within the liquid itself. This pressure is transmitted in all directions and increases with depth.

Liquids have little compressibility: water, for example, does not change its density appreciably unless subjected to pressures on the order of 100 bars. In the study of fluid dynamics, liquids are often treated as incompressible, especially when studying incompressible flow.

If a liquid is at rest in a uniform gravitational field, the pressure $\backslash \; p$ at any point is given by

where:

Note that this formula assumes that the pressure at the free surface is zero, and that surface tension effects may be neglected.

Objects immersed in liquids are subject to the phenomenon of buoyancy, which is also observed in other fluids, but is especially strong in liquids due to their high density.

The surface of a liquid behaves like an elastic membrane in which surface tension appears, allowing the formation of drops and bubbles. Capillarity, wetting, and ripples are another consequence of surface tension.
Viscosity measures the resistance of a liquid which is being deformed by either shear stress or extensional stress.

Thermodynamics of liquids

At a temperature below the boiling point, a liquid will evaporate until, if in a closed container, the concentration of the vapors belonging to the liquid reach an equilibrium partial pressure in the gas. Therefore no liquid can exist permanently in a complete vacuum. Liquids at their respective boiling point change to gases (except when superheating occurs), and at their freezing points, change to solids (except when supercooling occurs). Even below the boiling point liquid evaporates on the surface.

Liquids can display immiscibility. The most familiar mixture of two immiscible liquids in everyday life is the vegetable oil and water in Italian salad dressing. A familiar set of miscible liquids is water and alcohol. Liquid components in a mixture can often be separated from one another via fractional distillation.

Liquids generally expand when heated, and contract when cooled. Water between 0 °C and 4 °C is a notable exception.

Microscopic structure of liquids

Unlike crystalline solids, liquids exhibit a significant degree of atomic and/or molecular mobility. Strong forces of interaction (both repulsive and attractive) compete to bind the atoms of any solid object together firmly, while the bonds of the corresponding liquid will remain temporary in nature. This is what distinguishes the mechanical properties (e.g. rigidity and shear strength) in condensed matter between the liquid and solid state.