Liquid . Fluid whose volume is constant under conditions of constant temperature and pressure. The constituent particles are joined to each other by lower attractive forces than in solids, therefore, they can move freely, which determines their fluidity (as opposed to viscosity). This explains why liquids take the shape of the container that contains them.

This same fact means that in the absence of gravity, the shape that liquids acquire is spherical, since this minimizes surface tension, as a consequence of the application of the Hamiltonian principle, which says that every mechanical system evolves towards a minimum of energy. . This minimum surface tension means that the liquid in the absence of external forces tends to decrease its surface as much as possible for a given volume, the sphere being the most optimal shape.


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  • 1 General characteristics
  • 2 Description of liquids
    • 1 Form
    • 2 Changes of state
  • 3 Properties of liquids
    • 1 Viscosity
    • 2 Fluency
    • 3 Vapor pressure
    • 4 Other properties
  • 4 Reference
  • 5 Source

General characteristics

A liquid is made up of molecules that are in constant and disorderly motion, and each one collides billions of times in a very short time. But, the intense forces of attraction between each molecule , or hydrogen bonds called dipole-dipole, elude free movement, in addition to producing a closer proximity than that which exists in a gas between its molecules.

Liquids have characteristics that place them between the completely chaotic and disordered gaseous state, and on the other hand, the solid state of a (frozen) liquid is called ordered. Therefore we can mention the three states of water (universal liquid), solid , gaseous and liquid.

From which the following characteristics are concluded:

  • They have constant volume.
  • They are incomprehensible.
  • They have strong internal friction which is known by the name of viscosity.

Liquid Description

Liquids are made up of substances in a state of matter intermediate between solid and gaseous. The molecules of liquids are not as close as those of solids , but they are less separated than those of gases . Molecules in the liquid state occupy random positions that vary over time. Intermolecular distances are constant within a narrow range. In some liquids, the molecules have a preferred orientation, which causes the liquid to have anisotropic properties (properties, such as the index of refraction, which vary according to the direction within the material).

Liquids have surface tension and capillarity, they generally dilate when their temperature increases and lose volume when they cool, although under compression their volume is very little variable, unlike what happens with other fluids such as gases. Objects immersed in any liquid are subject to a phenomenon known as buoyancy.


Its shape is spherical if no external force acts on it. For example, a drop of water in free fall takes the spherical shape. [one]

As a fluid subject to the force of gravity, the shape of a liquid is defined by its container. In a liquid subject to gravity at any point in its bosom, there is pressure of equal magnitude to all sides, as established by Pascal’s principle.

State changes

Under appropriate temperature and pressure conditions, most substances can exist in a liquid state. When a liquid exceeds its boiling point it changes its state to gaseous, and when it reaches its freezing point it changes to solid . Although at atmospheric pressure , however, some solids sublimate upon heating; that is, they go directly from the solid state to the gaseous state. The density of liquids is usually somewhat less than the density of the same substance in the solid state. Some substances, like water, are denser in the liquid state.

By means of fractional distillation, the liquids can be separated from each other as each evaporates upon reaching their respective boiling points. The cohesion between the molecules of a liquid is not strong enough so that the surface molecules can evaporate.

Liquid properties


Liquids are characterized by a resistance to flow called viscosity . This means that to maintain speed in a liquid it is necessary to apply a force, and if that force ceases, the movement of the fluid eventually ceases.

The viscosity of a liquid increases with increasing number of moles and decreases with increasing temperature. Viscosity is also related to the complexity of the molecules that make up the liquid: it is low in liquefied inert gases and high in heavy oils . It is a characteristic property of all fluids (liquids or gases).

Viscosity is a measure of the resistance to displacement of a fluid when there is a pressure difference. When a liquid or gas flows, the existence of a stationary layer of liquid or gas is assumed to adhere to the surface of the material through which the flow occurs. The second layer rubs against the superficially adhered one and this second with a third one and so on. This friction between the successive layers is responsible for the opposition to the flow, or is responsible for the viscosity.

The viscosity is measured in poises, being a poise the viscosity of a liquid in which to slide a layer of a square centimeter of area at the speed of 1 cm / s with respect to another stationary one located at a distance of 1 cm the force was necessary of a dyna.

Viscosity tends to decrease in liquids with increasing temperature , although a few liquids increase in viscosity when heated. For gases the viscosity increases with increasing temperature.

The viscosity of a liquid is determined by means of a viscometer, among which the most widely used is that of Ostwald, [2] which is used to determine relative viscosity, that is, knowing the viscosity of a standard liquid, generally water.


Fluidity is a characteristic of liquids and / or gases that gives them the ability to pass through any hole or hole, however small it may be, as long as it is at the same or lower level of the container in which they are found (the liquid and gas), unlike the remaining state of aggregation known as solid .

Fluidity is the opposite of viscosity, both are related to temperature and pressure. The higher the temperature, the more fluid a liquid has and the less fluid a gas has.

Vapor pressure

Pressure of a vapor in equilibrium with its liquid form, the so-called vapor pressure, only depends on temperature ; its value at a given temperature is a characteristic property of each liquid.

So are the boiling point, the solidification point and the heat of vaporization (essentially, the heat necessary to transform a certain amount of liquid into steam).

Under certain conditions, a liquid can be heated above its boiling point; liquids in that state are called superheated. It is also possible to cool a liquid below its freezing point and then it is called a supercooled liquid.

Other properties

Liquids do not have a fixed form but they do have volume . They have variability of shape and very particular characteristics that are:

  • Cohesion: attractive force between equal molecules
  • Adhesion: attractive force between different molecules.
  • Viscosity: resistance manifested by a liquid to flow.
  • Surface tension: force that manifests itself on the surface of a liquid, by means of which the outer layer of the liquid tends to contain the volume of the liquid within a minimal surface.
  • Capillarity: ease of liquids to climb through tubes of very small diameters (capillaries) where the cohesion force is exceeded by the adhesion force.


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