If a gas is a mixture of compounds, it will be.... Over time....
As well, the kinetic molecular theory states that (4 things)
It will be homogenous, as all gases are miscible. Over time gravity causes denser gases to settle beneath less dense gases if the temp are low enough. A model that states that ideal gas molecules have zero volume, exert no forces other than repulsive forces due to collisions, gas molecules make completely elastic collisions, and the average kinetic energy of a gas molecule is direcly proportional to the temperature of the gas