Liquid Definition

It is one of the three states of aggregation of matter and is characterized by being a constant volume fluid (adaptable to the container that contains it) depending on the temperature and pressure to which it is subjected. As in the case of gases (and unlike solids), they do not have a fixed structure, which means that they do not have their own shape.

It is possible to study liquids based on five typical characteristics that help us understand their behavior, they are: Surface Tension, Viscosity, Density, Cohesion, Adhesion and Capillarity. In the following lines we explain what each of these terms refers to.

Surface tension

It is a phenomenon associated with the forces of attraction between molecules, which although they are weaker than in solids (for this reason they do not have their own shape) are strong enough to hold them together.

The molecules inside the liquid are attracted by forces in the same direction, but opposite directions that are equal to each other. Now, within the liquid there are only forces of attraction for the molecules that are below and on the sides, this gives rise to surface tension. From there arises the characteristic shape of the drop, since the liquid seeks to have the minimum exposed surface, due to its inward attraction.

In short, this surface tension acts to form a thin elastic layer that retains a certain rigidity and gives us an idea of ​​the magnitude of intermolecular forces that occur.

The surface tension value depends on the type of liquid and decreases with increasing temperature since, as the kinetic energy of the molecules increases, the cohesion forces decrease.

Goo

It is defined as the resistance of a fluid to flow. Unconsciously, dulce de leche comes to mind, a liquid more viscous than water and this property is easily identifiable.

In liquids, this property is closely related to the cohesive forces (which hold molecules together) and therefore viscosity measures the force required to move one layer of liquid over another.

A liquid could move freely inside a horizontal tube thanks to its inertia if it were not for viscosity, which requires an external force for movement to occur in any direction.

Currently, viscometers are used to define the value of this property and it is tabulated for different fluids depending on their temperature.

Density

It is the amount of matter contained in a given volume.

Like viscosity, its value depends on the temperature and pressure to which the fluid is subjected and, for its determination, hydrometers are used.

Cohesion Forces

It is the force of attraction that holds similar molecules together and measures the tendency to remain in a certain group of particles of the same substance.

If we look at the increase in cohesive forces, we would say that solids are those with the greatest forces of attraction between similar particles, followed by liquids and, finally, gases.

Adhesion Forces

Conversely to the previous case, the adhesion forces occur between disparate molecules, for example, between a liquid and a solid. When there is adhesion, we are witnessing a phenomenon whereby the surface between which there is adhesion presents a greater force of interaction than the cohesion force experienced by the fluid molecules among themselves.

We can easily remember a rainy afternoon, when the water falls on the glass, it experiences a force of adhesion.

However, the forces of adhesion and cohesion can coexist simultaneously. When a liquid adheres to a solid, adhesion forces are experienced between the two surfaces and cohesive forces are experienced to hold the liquid together.

capillarity

It is a phenomenon that allows the movement of a liquid inside a tube with a small hole called a capillary.

This phenomenon is widely used in industry, for example, for instrumentation design. Such is the case of fluid charges, which base their physical principle on a liquid vapor equilibrium of a mixture of alcohols through a capillary.

Both surface tension and the aforementioned forces give rise to capillarity. If we observe that a liquid is capable of ascending through a small diameter tube to a height greater than its free surface, it is because the liquid adheres to the capillary wall and the adhesion force is greater than the cohesion force.

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