Phase states of matter

Matter is the substance of which physical objects consist. Its physical state is described mainly by pressure and temperature. Under different physical conditions, matter may exist in four main phases: gas, liquid, solid, and plasma.

The particles of matter are in continuous and irregular motion known as thermal motion. The character of this motion and the interactions between particles differ between individual phases. In solids, for example, particles mainly vibrate and rotate around their equilibrium positions.

Gaseous phase

The gaseous phase may consist of individual atoms, such as in noble gases, elemental molecules such as oxygen (O₂), or compound molecules such as carbon dioxide (CO₂).

Gas particles are separated by relatively large distances. As a result, gases have low density and viscosity and are highly compressible. Interactions between gas particles are relatively weak, and gases are highly mobile and diffusible. They flow easily, occupy all available space and take the shape of their container.

Mixtures of gases are homogeneous. A common example is air, which consists of several different gases.

Ideal gas

An ideal gas is a simplified model of a gas. It assumes that:

  • the gas contains a very large number of molecules moving randomly,
  • the kinetic energy of the molecules is associated with their translational motion,
  • molecules interact only during brief, elastic collisions,
  • the distance between molecules is much greater than their diameter.


The state of an ideal gas is described by the ideal gas law:

pV = nRT

where:

  • p = pressure
  • V = volume
  • n = amount of substance
  • R = universal gas constant
  • T = absolute temperature

The universal gas constant is approximately : R = 8.31 J/(mol·K)


The ideal gas law can also be written using the total number of particles N and the Boltzmann constant k: pV = NkT

Several important gas laws follow from the ideal gas equation.

At constant temperature (T = constant), Boyle's law states : pV = constant

At constant pressure (p = constant), Charles' law states : V/T = constant

At constant volume (V = constant) : p/T = constant