Gases are fluid in the gaseous state. The distinguishing feature of liquid fluids is that when placed in a container they have the capacity to fully occupy it. Most known non-metallic chemical elements are found in their gaseous state at room temperature.

The molecules of the gas, when moving, collide with the other molecules and the walls of the container where they are, exerting a pressure, called gas pressure.

This pressure is related to gas volume and absolute temperature.

As the temperature rises, the gas molecules increase their agitation, causing more collisions.

By increasing the volume of the container, the molecules have more room to move, so collisions decrease, decreasing the pressure.

Using the principles of Newtonian mechanics it is possible to establish the following relationship:


p = pressure

m = gas mass

v = average velocity of molecules

V = gas volume.

Perfect or ideal gas

It is considered a perfect gas when the following characteristics are present:

  • the movement of molecules is governed by the principles of Newtonian mechanics;
  • the shocks between the molecules are perfectly elastic, that is, the amount of movement is conserved;
  • There is no attraction or repulsion between the molecules;
  • The volume of each molecule is negligible when compared to the total volume of the gas.

Kinetic energy of a gas

Due to collisions with each other and with the vessel walls, molecules change their velocity and direction, causing a variation of the kinetic energy of each one. However, the average kinetic energy of the gas remains the same.

Again using concepts of Newtonian mechanics it is established:


n = gas molar number (number of moles)

R = universal constant of perfect gases (R = 8,31J / mol.K)

T = absolute temperature (in Kelvin)

The number of moles of the gas is calculated using its molar mass, found in periodic tables and through the constant from Avogadro.

Using the ratio that in 1mol of molecules of a substance there is molecules of this substance.