# Partial pressure

Am I assuming right the pressure in a vessel holding two substances (both being in the both liquid and gas states) will be equal to the saturation prerssure of the one which has the higher SP ? That is, for substance A whose SP at a given temp. is 10 B, for instance, and substance B whose SP is 15 B, the partial P will be 15 B?

## Answers and Replies

Jano L.
Gold Member
Why would you think that ? That would suggest that the gas with higher saturation pressure somehow prevents the other gas from evaporating, but that is hard to believe.

Do you mean it is over 15 B ?

Jano L.
Gold Member
My guess would be that the gases will evaporate almost independently, so both gases will have their respective pressure as if they were alone in the vessel. Then total pressure will be the sum of the two partial pressures. However, even approximately true, some deviation from simple sum should be expected, since the two substances influence each other, more in the liquid and at least a little bit in the gas...

Redbelly98
Staff Emeritus
Science Advisor
Homework Helper
My guess would be that the gases will evaporate almost independently, so both gases will have their respective pressure as if they were alone in the vessel. Then total pressure will be the sum of the two partial pressures.
Yes, those were my thoughts as well.

However, even approximately true, some deviation from simple sum should be expected, since the two substances influence each other, more in the liquid and at least a little bit in the gas...
Hmm, you bring up a good point. If the liquids are not mixed, I expect the total pressure should still be the sum of the two vapor pressures. But if the liquids are mixed, I guess you could get some boiling point elevation, which suggests a lowering of the vapor pressure at a given temperature.

Just an educated guess on my part after reading your post. Somebody with a stronger chemistry background may know better.

Somebody with a stronger chemistry background may know better.
It is not chemistry but rather physics.