# At boiling point, is the total pressure twice the atmospheric pressure? [duplicate]

We know that one definition of boiling point is that it's the temperature at which the vapor pressure is same as the atmospheric pressure.

Assuming a closed container, does that mean that at this temperature, the total pressure exerted on the liquid is twice the atmospheric pressure, i.e., the vapor pressure plus the pre-existing atmospheric pressure?

edit: Based on some comments, it appears that closed container assumption is not very helpful. The questions still remains, even without that assumption: Does the total pressure become twice the atmospheric pressure at the surface?

• No, because at the expected "boiling point", 100°C, the atmospheric pressure will be two bars. In a closed vessel, water does not boil. – Karl Jul 7 '19 at 20:18
• First of all, the "duplicate" question has discussion that claims it is not always true that boiling doesn't happen in closed containers. And secondly, the closed container assumption is tangential to the question in the title. If closed container is a bad assumption, the question still remains: Does the total pressure become twice at BP temperature, in an open system? – Fi Zixer Jul 8 '19 at 1:43
• Closed vessel is not an assumption, it's the condition you have set in your question. Do you want it or not? – Karl Jul 8 '19 at 6:18
• The pressure in an open vessel is, quasi by definition, ambient pressure. If it was higher, water wouldn't boil at 100°C at ambient pressure, and here everything starts to get a bit illogical imo. – Karl Jul 8 '19 at 6:25

If you mix two gases of different pressure, the resulting gas does not have a pressure equal to the sum of the two pressures. Treating the gases as ideal gases, the pressure will be the average of the two pressures, weighted by the volumes. What is additive is the partial pressure of a gas mixtures, where the sum is the total pressure.

Closed container with constant volume

To simplify things, let's place a closed bottle filled with the liquid of interest into the container, adjust the pressure to 1 atm, and close the container. If we heat the container, the pressure will go up just because the gas temperature rises. If we open the bottle, some of the liquid will evaporate until the partial pressure is equal to the vapor pressure. Again, the container will have a pressure higher than atmospheric pressure. So the liquid will not boil when the temperature reaches the normal boiling point because the pressure will be above atmospheric pressure at that point, for the two reasons mentioned.

Closed container with variable volume

If the experiment is done in a closed, variable volume container that expands into the surrounding kept at 1 atm, the volume will increase as the temperature is raised (because the gas expands, and because the vapor pressure increases with increasing temperature). When the temperature reaches boiling temperature (slightly above, actually), the liquid will boil until it is all gone, and the volume will increase further during this boiling process. The partial pressure of the liquid will never reach 1 atm (because there are other gases present), so the vapor pressure will remain higher than the partial pressure throughout the boiling process.

Does the total pressure become twice the atmospheric pressure at the surface?

No. Either there is room to expand, and the gas produced by boiling will push away the other gas (and then quickly mix with it), or we keep the volume constant, and then we don't call it boiling (because the total pressure exceeds 1 atm, so the liquid will not boil. However, when you heat a liquid to the boiling point and let the system reach equilibrium, the partial pressure of that substance will be 1 atm, and if any other gases are present, the total pressure will be higher. How much higher depends on the starting conditions. If our starting condition is a completely filled closed bottle, with inert gas (e.g. nitrogen or air) already at the boiling point of the liquid and at 1 atm, when we open the bottle the total pressure will reach 2 atm.

• I think you need to define what you mean by "mix," since that is not necessarily clear to a beginner reader. – Zhe Jul 8 '19 at 11:59
• @Karsten Theis ,"When the temperature reaches boiling temperature (slightly above, actually)" is from your answer.A dought though,if liquid in a closed vessel reachs boiling point ,with pressure outside at 1 atm ,would the pressure in a closed container be < 1atm or >1 atm.Are we assuming that this closed container ,its boundries are insulated from surroundings? In such a case what will be the pressure inside ? – Chakravarthy Kalyan Jul 8 '19 at 13:26
• @ChakravarthyKalyan Could you ask a new question, and make clear what you are asking? If the container is constant volume, the outside pressure has no influence on the outcome. – Karsten Theis Jul 8 '19 at 13:59