A system of fixed volume contains two fluids as illustrated in the figure below, in equal volumes. Their pressures are both equal and high (much greater than atmospheric pressure).

Initially the liquid and gas are separated by a seal. Assuming the seal has negligible volume, the seal is instantaneously removed and the liquid and gas phase are hydraulically connected. Let's assume the gas behaves ideally, does not react with the liquid, and the equilibrium process is isothermal.

  1. Does the system's pressure decrease do to gas dissolving into the liquid?

After reading about Henry's Law, I come to the conclusion that it shouldn't. Insight from this law tells me only that the amount of gas that dissolves into the liquid increases with an increase in partial pressure of the gas.

One approach I thought about is using the law of conservation of energy as my argument for why the system pressure should remain constant. The law states that the total energy of an isolated system remains constant. The gas under pressure can be defined by its kinetic energy (I suppose the liquid could be too?). So, if the gas is dissolved by the liquid, wouldn't the momentum transfer between the gas and liquid maintain the system pressure to be constant?

  1. What processes take place once the seal is removed and the liquid and gas phase are hydraulically connected?
  2. How is the final (equilibrium) pressure of the system calculated?

This question stems from a real issue I am trying to understand. The liquid is odorless mineral spirits and the gas is nitrogen (just a FYI).

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    $\begingroup$ I believe that you'll need to consider both the solubility of the gas in the liquid (you have) and the evaporation of the liquid into the gas. $\endgroup$ Feb 12, 2015 at 1:57

1 Answer 1


Consider an extreme version. Ammonia gas in one, and water in another. This is a classic chemistry experiments called an Ammonia Fountain Pressure decreases substantially

  • $\begingroup$ So the gas dissolves into the liquid, reducing the moles of gas in the gaseous phase. By the ideal/real gas law the gas pressure would decrease, and in turn, the system pressure decreases. Where does the energy go? $\endgroup$
    – Armadillo
    Feb 12, 2015 at 17:18
  • $\begingroup$ IIRC dissolving ammonia in water releases energy. However, the dissolution is also a weak chemical reaction. But that is also likely true of many gases that dissolves to a significant extent. $\endgroup$
    – user56903
    Feb 12, 2015 at 18:20
  • $\begingroup$ releases energy in the form of temperature outside of the system? $\endgroup$
    – Armadillo
    Feb 12, 2015 at 18:22

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