The cohesive forces are intermolecular forces of attraction that hold them together with other molecules. Depending on the strength of the cohesive forces, a substance is in a solid, liquid or gaseous state. The value of the cohesive 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 cohesive forces is that they rapidly diminish as distance increases. So, cohesive forces are called attraction forces that occur between molecules of the same substance.
Rather, repulsive forces are those that result from the kinetic energy (energy due to motion) of particles. This energy causes molecules to be constantly in motion. The intensity of this movement is directly proportional to the temperature at which the substance is.
To cause a substance to change state, it is necessary to raise its temperature by means of heat transfer. This causes the repulsive forces of the substance to increase, which, if necessary, can 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 attractive forces that occur between adjacent particles of the same substance; on the other hand, adhesion is the result of the interaction that takes place between surfaces of different substances or bodies.
These two forces seem related in several physical phenomena that affect liquids; therefore, a good understanding of both is important.
Characteristics in solids, liquids and gases
in solids
In general, in solids, the cohesive forces are very high and occur intensely in the three directions of space.
Thus, if an external force is applied to a solid body, only small displacements of molecules occur between them.
Furthermore, when the external force disappears, the cohesive forces are strong enough to return the molecules to their original position, recovering the position prior to the application of the force.
in liquids
In contrast, in liquids, cohesive forces are high only in two spatial directions, while they are very weak between layers of fluid.
Thus, when a force is applied in the tangential direction of a liquid, that force breaks the weak bonds between the layers. This causes the layers of liquid to slide over each other.
Then, when the application of force ends, the cohesive forces don’t have enough strength to return the liquid molecules to their original position.
In addition, cohesion in liquids is also reflected in surface tension, caused by an unbalanced force directed into the liquid, acting on surface molecules.
Likewise, cohesion is also observed when the transition from liquid to solid state occurs, due to the effect of compression of liquid molecules.
in gases
In gases, the cohesive forces are negligible. In this way, gas molecules are in constant motion because, when appropriate, the cohesive forces are unable to hold them together.
Therefore, in gases, the cohesion forces can only be appreciated when the liquefaction process takes place, which occurs when the gas molecules are compressed and the attractive forces are strong enough for the state transition to occur. gaseous to the liquid state.
Examples
Cohesive forces are often combined with adhesion forces to give rise to certain physical and chemical phenomena. Thus, for example, cohesion and adhesion forces allow us to explain some of the most common phenomena that occur in liquids; This is the case of meniscus, surface tension and capillarity.
Therefore, in the case of liquids, it is necessary to distinguish between the cohesive forces, which occur between molecules of the same liquid; and adhesion, which occur between the molecules of the liquid and the solid.
Superficial 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.
Finally, surface tension occurs because the forces that occur on the liquid’s molecules are different on the liquid’s surface and those that occur on the inside.
Meniscus
The curvature that is created on the surface of liquids when they are confined in a container is called a meniscus. This curve is produced by the effect that the surface of the container that contains it exerts 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 is attractive – as is the case with water and glass – or repulsive, as is the case with mercury and glass.
Capillarity
Capillary action 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 increase of water inside the plants.
A liquid rises through the capillary tube when the cohesive forces are lower than those of adhesion between the liquid and the tube walls. In this way, the liquid will continue to rise until the surface tension value corresponds to the weight of the liquid contained in the capillary tube.
On the contrary, if the cohesive forces are greater than the adhesion forces, the surface tension will lower the liquid and the shape of its surface will be convex.
