Gases (for example helium, air, carbon dioxide, steam,...) normally flow easier and intermix easier than liquids.
Gases likewise do not have their own shape. Unlike liquids, gases occupy the whole available space, therefore they do not have free surface. Gas in the vessel exerts pressureon each wall of the vessel, including the upper wall with practically the same pressure. It is impossible for gas to occupy only the lower part ofthe vessel and leave vacuum above.
