Molecular kinetic theory: the 3 states of matter

It is said that the entire universe is made up of matter and that when it changes, energy is generated. And as is normal, the curious nature of the human being has led us to wonder on many occasions what all this matter is made of. Throughout history, different models have been devised to explain this, one of them being molecular kinetic theory.

According to this model, matter would be constituted by a fundamental unit that cannot be appreciated with the senses, I am talking about the atom. In turn, the atoms group together to form molecules.

To take a classic example, the water molecule is structured with one oxygen atom and two hydrogen atoms (H2O). But the kinetic theory not only postulates this, but also because they exist the three fundamental states of matter: solid, liquid and gas.

• You may be interested: "The 5 types of chemical bonds: this is how matter is composed

The origin of the kinetic theory

Until arriving at the formulation of this model, different events took place that allowed the bases to be given to offer this theory.

To get started, the concept of the atom was born in ancient Greece, under the atomistic school, whose disciples spread the idea that the atom is the indivisible unit that forms all matter in the universe. Democritus was one of the greatest exponents of him, but his proposals clashed directly with the ideas of Aristotle, which dominated the time, so they went unnoticed.

It was not until the early nineteenth century that the idea of ​​the atom reappeared in the field of science, when John Dalton postulated the atomic theory, indicating that every substance is made up of atoms.

Prior to this, Daniel Bernoulli in 1738 argued that the gases were made up of molecules that collide with each other and with the surfaces, generating the pressure that is felt. Following the emergence of atomic theory, it is now recognized that these molecules are made up of atoms.

The molecular kinetic theory is born from a set of studies that were carried out mainly in gases, and whose final conclusion was similar. Some of the notable works are those made by Ludwig Boltzmann and James Clerk Maxwell.

• Related article: "The 9 postulates of Dalton's atomic theory"

The argument

This kinetic molecular theory postulates that matter is made up of a set of particles that are known as atoms or their molecules, that are constantly in motion. As they do not stop moving, sooner or later they collide with another atom or against a surface.

This collision is kinetic, in other words, energy is transferred without losses, so the atom when colliding is thrown in the other direction at the same speed, without stopping the movement. The kinetic energy that is generated in the collision is translated into the pressure that is felt.

Difference between states of matter

Although the molecular kinetic theory was born from the study of the gaseous state, since there were many studies on this that allowed to write the ideas, it also serves to explain the constitution of liquids and solid. What's more, it offers a way to see differences between different states of matter.

The key point lies in the degree of movement of atoms. Matter is made up of a set of particles that are in constant motion; In a gas, the atoms are free and move in a linear way through all the available space, demonstrating the characteristic of gases to always occupy all the space they have.

In the case of liquids, the distance between the atoms is not that greatIf not, they are closer together, although they continue to move with less speed. This explains that a liquid occupies a fixed volume, but can expand on a surface.

By last, in the solid state the atoms are very close, without a free movement although they vibrate in place. Therefore, solids occupy a specific space and do not vary in volume over time.

According to molecular kinetic theory, the force that binds atoms together is known as cohesion force. Its name is given because the solids that have a greater presence in these unions, that is, it is more cohesive than a liquid or a gas.

The importance of this model

What is interesting about this theory is how it relates the existence of the atom to measurable physical properties, such as pressure or temperature. In addition, it has a correlation with the mathematical formulas of the ideal gas laws.

I will not go into much detail about this, but for example, it agrees with the formulas that indicate that the higher the temperature, the atoms have a higher speed. It is easy to understand, for an ice to go to liquid and then to vapor you need to apply heat. As the temperature rises, the H2O molecules gain speed and break the cohesion forces, changing the state of matter.