# Why does the partial pressure of H2O stay the same

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In these slides they show the partial pressure of the H2O gas not changing when the enternal pressure on the entire gas is increased. Why is this the case? I know it condenses to maintain the same partial pressure, but couldn't the partial pressure of the gas just increase proportionally to the increase in pressure?

According to Henry's law, K(H2O)x(H2O)=p(H2O), and since temperature remains the same K is constant. But couldn't x and p increase?

The other reason that I thought of was that H2O starts out at a pressure and temperature at a phase transition so it only has one degree of freedom, so since temperature is constant the pressure must remain the same as well. But the problem I have with this argument is that when I increase the overall pressure, if my partial pressures increase proportionally then would I be pushed up to a pressure where the gas liquefies? Such that all the vapour just liquefied and only gas remains?

russ_watters
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In these slides they show the partial pressure of the H2O gas not changing when the enternal pressure on the entire gas is increased. Why is this the case? I know it condenses to maintain the same partial pressure, but couldn't the partial pressure of the gas just increase proportionally to the increase in pressure?
I'm having trouble following what your concern is. you seem to already know what happens...are you just looking for a specific reason why it condenses or why it can't just increase in pressure? Saturation pressure is the maximum pressure of vapor possible a a certain temperature. Increase the pressure and you are basically pushing molecules into a liquid. I'm thinking maybe your real question is why does saturation pressure even exist? Or why do substances have more than one possible state? Or is it about the mechanism by which the condensation occurs?

[edit: I said vapor pressure when I meant saturation pressure.]

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As Russ said, It sounds as if you understand what's going on. My sense is that you're mixing 'dynamic' and 'static' behavior. It is true that compressing a gas (which contains an equilibrium-quantity of water vapor) will result is an elevated water vapor pressure, initially. Assuming that the temperature didn't change, vapor will return to the liquid phase at a rate determined by a number of physical factors. Analysis of that transition (and the associated rates) is beyond the scope of your slides. The take-home message is: the steady state partial pressure of water is a function of temperature (only). The pressure of 'other gases' doesn't matter.

russ_watters
I'm having trouble following what your concern is. you seem to already know what happens...are you just looking for a specific reason why it condenses or why it can't just increase in pressure? Saturation pressure is the maximum pressure of vapor possible a a certain temperature. Increase the pressure and you are basically pushing molecules into a liquid. I'm thinking maybe your real question is why does saturation pressure even exist? Or why do substances have more than one possible state? Or is it about the mechanism by which the condensation occurs?

[edit: I said vapor pressure when I meant saturation pressure.]
As Russ said, It sounds as if you understand what's going on. My sense is that you're mixing 'dynamic' and 'static' behavior. It is true that compressing a gas (which contains an equilibrium-quantity of water vapor) will result is an elevated water vapor pressure, initially. Assuming that the temperature didn't change, vapor will return to the liquid phase at a rate determined by a number of physical factors. Analysis of that transition (and the associated rates) is beyond the scope of your slides. The take-home message is: the steady state partial pressure of water is a function of temperature (only). The pressure of 'other gases' doesn't matter.

I think I understand what's going on now. The pressure just stays at 0.031 atm because that's its vapour pressure so it can't go any higher than that at 298K.

However, I have another question regarding this example now. For example, if the air added undergoes a phase transition at 1.5atm. And we were to start compressing the gas to 2atm. The water would stay decrease from 0.031atm, slightly dropping actually because the air would dissolve slightly and it will continue to decrease in pressure as we continue to compress the gas since the water's mole fraction in the liquid would keep decreasing. But at 1.5atm, the air wouldn't be able to increase its pressure anymore because wouldn't it just liquefy. In this case, what will happen to equalize the external pressure? At this point, the mole fraction of water would rapidly drop and so the partial pressure of the water would drop even more. So wouldn't the internal pressure be 1.5atm + (<0.031atm)?

Or is there something wrong with the system I'm describing here?

Thanks for the help!