Ideal gases

Ideal gases

The concept of ideal gas represents a theoretical model that has allowed scientists to better understand the behavior of gases under different conditions. This is because, before they could tackle the complexity of real gases, scientists needed a simplified model, an “ideal,” to have a basis from which to start. In this context, ideal gases have become the archetype for studying the fundamental properties and laws of gases, offering a clear and understandable vision of an otherwise very complex world to manage.

What are ideal gases?

Ideal gases are a theoretical concept, a simplification of the behavior of real gases. This model assumes that the particles of a gas have no volume and that there are no attractive forces between them, which, of course, is not true for real gases. But why call them “ideal”? The name derives precisely from the fact that they represent an “ideal” or “perfect” condition in which gases would behave according to precise mathematical laws without the complications introduced by interactions between particles.

The birth of the concept of ideal gas dates back to the 17th century, but it was in the 18th and 19th centuries that scientists such as Boyle, Charles, and Avogadro formulated the laws describing the behavior of gases. These laws, when combined, lead to the ideal gas equation of state, a mathematical formula that relates pressure, volume, temperature, and quantity of gas.

The main characteristics of ideal gases can be summarized as follows:

  1. Non-interacting molecules: It is assumed that there are no forces of attraction or repulsion between the molecules of an ideal gas.
  2. Negligible molecular volume: The molecules of an ideal gas are considered point-like, meaning that their volume is negligible compared to the total volume of the gas.
  3. Kinetic energy: The energy of the molecules of an ideal gas depends only on the temperature.
  4. Compressibility: Ideal gases are highly compressible and expand to occupy all available volume.

Despite its simplifications, the ideal gas model has had a fundamental impact on the development of thermodynamics and the understanding of the macroscopic behavior of gases.

Properties of ideal gases


The properties of an ideal gas depend on the behavior of its particles. They constitute matter in the gaseous state and possess kinetic energy (energy of movement of the same), which prevails over the forces of mutual attraction. This means that, since they are not inclined to attract each other, they leave room for the entire gas to occupy all the available space. Gas therefore have neither their own shape nor volume; an ideal gas will tend to take on the shape of the closed container in which it is contained.

State law of ideal gases



All ideal gases have similar physical properties. Their behavior in relation to variations in pressure, volume, and temperature can be described on the basis of the following state law:

Kelvin degrees

To express the temperature of a gas , different units of measurement can be used . Generally, we are usually more familiar with degrees centigrade (or Celsius) but in the equation of state of perfect gases the temperature must always be considered in degrees Kelvin to avoid making calculation errors. Let us therefore remember that the conversion between the two units of measurement is as follows:


So, given a temperature in degrees Celsius, it will be enough to add the constant 273.15in order to obtain the value expressed in Kelvin degrees.

The number of moles of ideal gases

In the equation of state of ideal gases, a quantity appears that we are not used to dealing with: the number of moles . This is a quantity that expresses the number of particles present within the gas being analysed.

Specifically, one mole corresponds to a fixed number of atoms or molecules , called Avogadro’s number and equal to6.022⋅1023. This constant is used to be able to more easily describe the very high number of atoms or molecules present in a gas. In fact, think about how it is much simpler to state that “a given gas is made up of 2 moles of atoms” than to specify that “there are12.044⋅1023atoms within it.

Consequences of the state law of ideal gases

The state law of ideal gas allows us to reach different conclusions regarding the behavior of their physical properties. By observing the terms that appear in the relationship we can in fact deduce that there are relationships of direct and inverse proportionality between the quantities described. Furthermore, by imagining fixing one of the physical properties in play, we can further investigate the behavior of the remaining ones.

Starting from this concept it is possible to define further physical laws, which can be obtained from the equation of state of ideal gases.

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