Our teacher while teaching thermodynamics talked about free expansion, isothermal free expansion and adiabatic free expansion.

But for isobaric free expansion, he said that's not possible (it's not a thing) because when the pressure is a constant, volume of gas can't be increased.

But doesn't isobaric expansion mean just that, the volume of gas increasing without pressure changes.

So, is isobaric free expansion a thing? Yes/No? Why?

  • 4
    $\begingroup$ A free expansion implies no external pressure holding back the expansion. $\endgroup$
    – Buck Thorn
    May 21 at 6:38

1 Answer 1


If the volume of gas increases, but pressure stays the same, then something must be steadily supplying heat.

PV = nRT yields T = PV/nR

If P stays the same, and V increases, so must T.

This is obviously not a system in equilibrium.

A simple example would be a flask with a rubber stopper and glass tube, in which one inserts a drop of water half-way down the tube. The pressure in the flask equals current atmospheric pressure, ignoring the minuscule weight of the droplet. Warm the flask in your hand and the drop ascends: temperature increased, volume increased, pressure unchanged (still balances atmospheric).


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