Properties and Behavior of Liquids and Fluids

Properties and Behavior of Liquids

Fluidity is a property of liquids and gases that gives them the ability to pass through openings smaller than themselves, provided the opening is at the same level or lower than the container in which the fluid resides. This behavior distinguishes fluids from the solid state of matter.

Liquid State and Shape

Liquid is one of the four states of matter. A liquid is a fluid whose volume remains essentially constant under given temperature and pressure conditions, and its shape conforms to the container that holds it. A free droplet, when not acted on by external forces, tends toward a spherical shape due to surface tension. ^[1] In a gravitational field, a liquid instead conforms to the shape of its container.

Pressure in Static Liquids

A fluid at rest exerts pressure on the container with equal magnitude in all directions. If a liquid is at rest, the pressure at a depth is given by:
p = p₀ + ρ g h
where p₀ is the reference pressure at the surface, ρ is the fluid density, g is the acceleration due to gravity, and h is the depth below the surface.

Surface Tension, Capillarity and Buoyancy

Liquids exhibit surface tension and capillary action. They usually expand when their temperature increases and contract as they cool. Objects immersed in a liquid are subject to buoyancy, the upward force that acts opposite to gravity and depends on the displaced fluid volume and density.

Molecular Structure and Phase Changes

The molecules in a liquid occupy random positions that vary over time; intermolecular distances remain relatively constant within a narrow range. When a liquid is heated past its boiling point, it transitions to the gaseous state; when it is cooled to its freezing point, it becomes a solid. At appropriate temperature and pressure conditions most substances can exist in the liquid form.

Separation, Distillation and Density

Through fractional distillation, components of a liquid mixture can be separated by evaporating components in sequence as each reaches its respective boiling point. Cohesion between molecules in a liquid is not large enough to prevent surface molecules from evaporating.

Liquid substances occupy a state of matter intermediate between solids and gases. The molecules in liquids are not as closely packed as in solids, but are less separated than in gases. In some liquids the molecules have a preferred orientation, which produces anisotropic properties (for example, a refractive index that varies with direction within the material). At atmospheric pressure some solids sublimate when heated, passing directly from the solid to the gaseous state (see evaporation).

The density of liquids is usually somewhat less than the density of the same substance in solid form, although exceptions exist: some substances (for example water) are denser in the liquid state than in the solid state.

Key Properties

• Fluidity: ability to flow through openings
• Constant volume: under given conditions of temperature and pressure
• Surface tension & capillarity: govern droplet shape and rise in narrow tubes
• Buoyancy: upward force on immersed objects
• Phase transitions: boiling, freezing, evaporation, sublimation