Vapor pressure + air pressure?

Click For Summary

Discussion Overview

The discussion revolves around the relationship between vapor pressure and air pressure in a sealed vessel containing water. Participants explore how these pressures interact when the water is heated, particularly at room temperature and at 100°C, and whether they combine to yield a total pressure in the system.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • James questions whether the vapor pressure should be combined with the air pressure to determine the total pressure in a sealed test tube containing water.
  • James proposes that at room temperature, the pressure inside the sealed test tube would be 1 bar and wonders if heating the water to 100°C would result in a total pressure of around 2 bar due to the addition of vapor pressure.
  • UltrafastPed references Dalton's Law of Partial Pressures, suggesting that the partial pressures of air and water vapor combine to give a total pressure.
  • James asks if deviations from Dalton's Law at high pressures would significantly affect the total pressure calculation when considering air and water vapor pressures.
  • Another participant confirms that the partial pressure of air would be slightly less than 1 bar when the test tube is sealed due to the equilibrium vapor pressure of water at the given temperature.
  • It is noted that the total pressure would be greater than 2 bar when the air pressure increases upon heating the test tube.
  • There is a disagreement regarding whether the total pressure could be closer to 1 bar due to the air pressure affecting the vaporization of water at the given temperature, with a participant asserting that this is not the case.

Areas of Agreement / Disagreement

Participants express differing views on the interaction between vapor pressure and air pressure, particularly regarding the total pressure in the system. While some agree on the application of Dalton's Law, there is no consensus on the implications of air pressure on vaporization and total pressure calculations.

Contextual Notes

Participants acknowledge that real gases may deviate from Dalton's Law, particularly at higher pressures, but the extent of this deviation in the context of the discussion remains unresolved.

jmsbot
Messages
2
Reaction score
0
Hi, could someone please tell me if the vapor pressure should be combined with the air pressure in a sealed vessel to give the total pressure? For example, if I put room temperature water in a test tube (at sea level) with some head space for air and then seal it, the pressure inside would be 1 bar, would it not? If I then heated the water inside to 100C, the vapor pressure should equal 1 bar also, shouldn't it? Does that mean that the air pressure and the vapor pressure combine to give a total pressure of around 2 bar? Or is the total pressure closer to 1 bar because the air pressure decreases the amount of water that can vaporize at the given temperature? I apologize for my somewhat rudimentary questions and many thanks for your help

James
 
Last edited:
Physics news on Phys.org
Thanks, UltrafastPed. So according to Dalton's Law, the partial pressure of the air at 1 bar and the partial pressure of the water vapor at 100C, combine to give a total pressure of 2 bar?

If "Dalton's law is not exactly followed by real gases" and "those deviations are considerably large at high pressures", would 2 bar be considered high enough (for air + water) to cause much of a deviation? Or will it still be relatively close to 2 bar? Thanks again for your help.
 
Last edited:
If those are the correct partial pressures, then yes.
 
jmsbot said:
Hi, could someone please tell me if the vapor pressure should be combined with the air pressure in a sealed vessel to give the total pressure? For example, if I put room temperature water in a test tube (at sea level) with some head space for air and then seal it, the pressure inside would be 1 bar, would it not?

Yes. Also, as you are in the process of sealing the test tube, the water vapor partial pressure in the head space would attain the equilibrium vapor pressure of water at the test tube temperature, so that the partial pressure of the air in the head space would be slightly less than 1 bar.
If I then heated the water inside to 100C, the vapor pressure should equal 1 bar also, shouldn't it?
Yes.
Does that mean that the air pressure and the vapor pressure combine to give a total pressure of around 2 bar?

Roughly yes. Of course, the air partial pressure would also increase when you heat the test tube, so the total pressure would be greater than 2 bar.
Or is the total pressure closer to 1 bar because the air pressure decreases the amount of water that can vaporize at the given temperature?

No
 

Similar threads

  • · Replies 13 ·
Replies
13
Views
4K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 9 ·
Replies
9
Views
4K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 9 ·
Replies
9
Views
4K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 0 ·
Replies
0
Views
1K
  • · Replies 1 ·
Replies
1
Views
10K
  • · Replies 40 ·
2
Replies
40
Views
4K