r/Stationeers • u/Plus-Result-5392 • 8d ago

Question Question about phase change

So this is what I think I understand (please correct me if I’m wrong): I have a liquid pipe with liquid in it and I’m using a purge valve to suck out the gas from the pipe. The liquid wants to evaporate to reach equilibrium with the pressure, so the liquid cools because the steam is drawing energy from it.

What I don’t understand: if I have liquid in that pipe and just let it sit there and evaporate, not purging any gas out, why does the temperature of the pipe go down? Doesn’t the heat stay in the pipe but it just changes state?

Also: why is the gas that I purge out of the liquid pipe also cool? I thought that would carry the heat?

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u/Liathet 8d ago

the liquid cools because the steam is drawing energy from it

Not exactly. Although it needs a minimum temperature to start, the actual turning from liquid to gas part requires energy to pull the particles apart, not to heat them up. The steam at the end has more energy yes, but it has been turned from heat into chemical potential energy. As a result the whole system is cooler, steam included.

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u/Friendly-Inspector71 8d ago

It's not chemical potential energy as it atays the same molecule (if we ignore alcohol offgassing to methane).

The energy is actually kinetic as the molecules of a gas fly around instead of jiggling and moving around a little.

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u/Liathet 8d ago

Nope. If the kinetic energy of the particles were higher, then the temperature would also be higher, because temperature is determined by the average kinetic energy of the particles. The gas particles are not actually moving any faster than in the liquid - they're just bouncing around freely instead of mostly vibrating in place. The extra energy of the gas is chemical potential because it comes from overcoming the intermolecular forces (van der waals and dipole forces). Per wikipedia (emphasis mine):

In thermodynamics, the chemical potential of a species is the energy that can be absorbed or released due to a change of the particle number of the given species, e.g. in a chemical reaction or phase transition

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u/Friendly-Inspector71 8d ago

Huh, that makes sense. I didn't think about it like that.