COHESION FORCE: CHARACTERISTICS IN SOLIDS, LIQUIDS, GASES - PHYSICAL - 2025

The cohesive forces are intermolecular forces of attraction that hold them together with other molecules. Depending on the intensity of the cohesion forces, a substance is in a solid, liquid or gaseous state. The value of the cohesion forces is an intrinsic property of each substance.

This property is related to the shape and structure of the molecules of each substance. An important characteristic of cohesion forces is that they decrease rapidly with increasing distance. Then, cohesion forces are called the attractive forces that exist between the molecules of the same substance.

On the contrary, the repulsive forces are those that result from the kinetic energy (energy due to the movement) of the particles. This energy causes the molecules to be constantly in motion. The intensity of this movement is directly proportional to the temperature at which the substance is.

To cause the change of state of a substance it is necessary to raise its temperature through the transmission of heat. This causes the repulsive forces of the substance to increase, which in the case may end up assuming that the change of state takes place.

On the other hand, it is important and necessary to distinguish between cohesion and adhesion. Cohesion is due to the attractive forces that occur between adjacent particles of the same substance; instead, adhesion is the result of the interaction that occurs between surfaces of different substances or bodies.

These two forces appear related in various physical phenomena that affect liquids, so a good understanding of both is important.

Characteristics in solids, liquids and gases

In solids

In general, in solids the cohesion forces are very high and they occur intensely in the three directions of space.

In this way, if an external force is applied to a solid body, only small displacements of the molecules take place between them.

Furthermore, when the external force disappears, the cohesion forces are strong enough to return the molecules to their original position, recovering the position prior to the application of the force.

In liquids

On the contrary, in liquids the cohesion forces are high only in two of the spatial directions, while they are very weak between the fluid layers.

Thus, when a force is applied in a tangential direction on a liquid, this force breaks the weak bonds between the layers. This causes the layers of the liquid to slide over each other.

Later, when the application of the force is finished, the cohesion forces are not strong enough to return the molecules of the liquid to their original position.

Furthermore, cohesion in liquids is also reflected in surface tension, caused by an unbalanced force directed towards the interior of the liquid, acting on the surface molecules.

Likewise, cohesion is also observed when the transition from the liquid state to the solid state occurs, due to the effect of the compression of the liquid molecules.

In gases

In gases the cohesion forces are negligible. In this way, the gas molecules are in constant motion since, in their case, the cohesion forces are unable to keep them bound together.

For this reason, in gases the cohesion forces can be appreciated only when the liquefaction process takes place, which takes place when the gaseous molecules are compressed and the attractive forces are strong enough for the state transition to occur. gaseous to liquid state.

Examples

Cohesion forces often combine with adhesion forces to give rise to certain physical and chemical phenomena. Thus, for example, cohesion forces together with adhesion forces make it possible to explain some of the most common phenomena that occur in liquids; This is the case of the meniscus, surface tension and capillarity.

Therefore, in the case of liquids, it is necessary to distinguish between the cohesion forces, which occur between the molecules of the same liquid; and those of adhesion, which occur between the molecules of the liquid and the solid.

Surface tension

Surface tension is the force that occurs tangentially and per unit length at the edge of the free surface of a liquid that is in equilibrium. This force contracts the surface of the liquid.

Ultimately, surface tension occurs because the forces in the molecules of the liquid are different on the surface of the liquid than they are on the inside.

Meniscus

Meniscus is the curvature that is created on the surface of liquids when they are confined in a container. This curve is produced by the effect that the surface of the container that contains it has on the liquid.

The curve can be convex or concave, depending on whether the force between the molecules of the liquid and those of the container are attractive - as is the case with water and glass - or are repulsive, as occurs between mercury and glass..

Capillarity

Capillarity is a property of fluids that allows them to ascend or descend through a capillary tube. It is the property that allows, in part, the ascent of water inside the plants.

A liquid rises up the capillary tube when the forces of cohesion are less than those of adhesion between the liquid and the walls of the tube. In this way, the liquid will continue to rise until the value of the surface tension equals the weight of the liquid contained in the capillary tube.

On the contrary, if the cohesion forces are greater than the adhesion forces, the surface tension will lower the liquid and the shape of its surface will be convex.

References

1. Cohesion (Chemistry) (nd). In Wikipedia. Retrieved on April 18, 2018, from en.wikipedia.org.
2. Surface tension (nd). In Wikipedia. Retrieved on April 18, 2018, from en.wikipedia.org.
3. Capillarity (nd). In Wikipedia. Retrieved on April 17, 2018, from es.wikipedia.org.
4. Ira N. Levine; "Physicochemistry" Volume 1; Fifth edition; 2004; Mc Graw Hillm.
5. Moore, John W.; Stanitski, Conrad L.; Jurs, Peter C. (2005). Chemistry: The Molecular Science. Belmont, CA: Brooks / Cole.
6. White, Harvey E. (1948). Modern College Physics. van Nostrand.
7. Moore, Walter J. (1962). Physical Chemistry, 3rd ed. Prentice Hall.