# How do you define volume for gases? [closed]

Straight forward question. If atoms forming a gas float far from each other, how can we assign a volume to it? if it is in a container, perhaps we'd say the atoms, or molecules, reach every corner and thus its volume is the container's, but what about when it's in the air (think vacuum)? Do we consider a closed surface that contains the outermost atoms and the volume it encloses is the gas's?

• Unclear what you are asking. Gases in the air? Air is a gas, consists of molecules. – Pieter Sep 16 '19 at 12:46
• If atoms forming a gas float far from each other, how can we assign a volume to it? The gas mingles with the air around it because of diffusion. W/o sharp boundaries no volume can be defined. – Gert Sep 16 '19 at 14:01
• @Pieter think vacuum. – Luyw Sep 16 '19 at 15:15

If you are considering a gas/partial vacuum that is not confined in a well-defined 'container', you would presumably want use specific volume (aka inverse density) instead. I.e. consider the amount of volume occupied per unit mass of the substance.

• is there a reference distance between two immediate molecules (consider they're at the edge of the "volume") such that if surpassed one of them is no longer considered part of the volume? – Luyw Sep 16 '19 at 17:59
• There is the Knudsen Number, which compares the mean free path of molecules to a representative distance scale. As it approaches 1, the continuum hypothesis breaks down and so molecules can no longer be considered to be part of a continuum. – Time4Tea Sep 16 '19 at 18:42

Because gas is compressible, you must specify its pressure along with the volume it happens to occupy in order to properly define its state. And since heating a gas causes its pressure to increase, a complete description of any gas will necessarily include calling out its temperature as well.

Since more gas atoms in a fixed volume will exert more pressure on the thing containing them, a complete account of the state of a parcel of gas also includes a count of how many atoms of it there are inside that container.

This is summarized in an equation called the ideal gas law:

pressure x volume = number of atoms x a constant x temperature.

• I can't see how this relates to a gas in a vacuum. Can you please put me on track? – Luyw Sep 16 '19 at 18:02
• it does not relate to gas in a vacuum. I guess I do not understand then what exactly you are asking, because a gas cannot have a "pressure" in a vacuum, because the pressure in a vacuum is identically zero. – niels nielsen Sep 16 '19 at 23:45