Answer:
A gas expands spontaneously to fill its container.
Step-by-step explanation:
The molecules of a gas are cluttered and distant one from other, so when gas is put in a container, the molecules occupy all the space, so it expands spontaneously, and the volume is always equal to the volume of the container.
By the equation of the ideal gas PV = nRT, we can notice that pressure and volume are indirectly proportional, so when pressure is applied to gas it compress, and the volume decreases.
A gas mixture is always homogeneous, because of the distance between the molecules of the compounds, so it's impossible to determinate the phases.