An ideal gas is a hypothetical gas in which molecules occupy 'negligible space' and have 'no interactions', and which consequently obeys the gas laws exactly.

pV = nRT

where:

p = the pressure of the gas

V = the volume of the gas

n = the number of moles of the gas

R = the gas constant

T = the absolute temperature of the gas (the temperature in degrees kelvin)

The theory for ideal gases makes the following assumptions:

The gas consists of very small molecules (spoken of as particles), each of which has an identical mass.

The number of molecules is so large that the maths of statistics can be used.

The molecules are in constant, random motion. The rapidly moving particles constantly collide with each other and with the walls of the container with no preferred direction. In reality there is a preferred direction due to gravity and attraction between them.

The collisions of gas particles with the walls of the container holding them are perfectly elastic (kinetic energy is conserved).

The interactions between molecules are negligible. They exert no forces on one another except during collisions. Attractions between gas molecules in a real gas result in a lower gas volume at constant pressure than would be expected of an ideal gas, as they pull the particles closer. The attractive forces also decrease the force of collisions between the molecules and container walls, therefore reducing the pressure compared to an ideal gas prediction.

The total volume of the individual gas molecules added up is negligible compared to the volume of the container. Another way of saying this is that the average distance separating the gas particles is relatively large compared to their size.

The molecules are perfectly spherical in shape, and elastic in nature and tiny (particular).

The average kinetic energy of the gas particles depends only on the temperature of the system.

Relativistic effects are negligible (relativity comes into play when speed approach the speed of light - ignore this). In other words Newtonian mechanics apply!

The time during collision of molecule with the container's wall is negligible as comparable to the time between successive collisions.

An ideal gas does not exist - the nearest we have to it is helium remote from its liquefying point.