On the existence of ideal gases

Question

I am following Statistical Mechanics course at the university, and my professor came up with this statement:

Ideal gas model is nothing but a fictitious ideal system that doesn't really exist in nature. Sure we can approximate many gases to ideal gases just as a suitable fast approximation but it's anyway wrong.

The most similar thing to an ideal gas is gas of electrons, but even in that case, it wouldn't be ideal.

Question: Is this true? I mean, are ideal gases really inexistent? Why a "gas of electrons" is the closes system to an ideal gas?

Fictitious ideal systems are how we understand most natural phenomena. — Solomon Slow, Apr 15 '18 at 16:40
Possible duplicate of Why do real gases deviate from ideal behavior above the Boyle temperature? — John Rennie, Apr 15 '18 at 16:40
I guess you professor is also against using geometry because a perfect circle, square, rectangle, angle, etc is nothing but a fictitious ideal system that does not really exist in the real world. Sure we can approximate many geometric entities to ideal entities just as a suitable fast approximation, but it's anyway wrong. So stop using geometry immediately. It's useless. Does this make sense to you as well? — Chet Miller, Apr 16 '18 at 10:47
Possibly related: https://physics.stackexchange.com/q/272820/59023 — honeste_vivere, Apr 18 '18 at 14:09

ACuriousMind · Answer 1 · 2018-04-15T17:39:27.533

Of course this is true, just like there is no such thing as a "frictionless surface" or an "ideal wire" (well, not counting possibly superconductors). Physical models of real situations are almost always approximative in some sense or another.

If you look at the definition of an ideal gas from Wikipedia:

An ideal gas is a theoretical gas composed of many randomly moving point particles whose only interactions are perfectly elastic collisions [emphasis mine].

and then compare it to the real world, you'll certainly notice that the particles that make up "gases" tend to have some mode of interaction other than such collisions, but if these interactions have neglegible effect (because they are weak, rare, or some combination thereof) then you'll get away with using the ideal gas model just fine.

Note also that for situations where the small errors incurred by use of the ideal gas approximation cannot be ignored, there are other more complex forms of the law you can use which satisfactorily take into account those second and third order effects. — niels nielsen, Apr 15 '18 at 17:46

score 2 · Answer 2 · answered Apr 15 '18 at 18:58

Ideal gases are of course an approximation; an "ideal" in fact. A definition:

ADJECTIVE 1 Satisfying one's conception of what is perfect; most suitable.

‘the swimming pool is ideal for a quick dip’ ‘this is an ideal opportunity to save money’

2 Existing only in the imagination; desirable or perfect but not likely to become a reality.

‘in an ideal world, we might have made a different decision’

2.1 Representing an abstract or hypothetical optimum. ‘mathematical modelling can determine theoretically ideal conditions

A gas of electrons is ideal in the sense of point-like particles, but certainly not ideal in the sense of those particles being non-interacting. Compressing a gas of electrons will take more work than implied by $(nRT)dV$ because of their mutual Coulomb interactions.

On the existence of ideal gases

2 Answers2