Boiling Point of water in a vacuum

[betterfuture]

So boiling point is when the vapor P of a liquid = atm pressure. That being said, if a closed container of water is put in a vacuum, how is it possible for water to boil at that temperature, let's say, room temperature. This makes no sense. There is no Pressure above the vapor Pressure. How is it boiling? Anybody have any helpful links or videos to explain this?

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[aldol16]

I don't understand your question. If you take a container of water at atmospheric pressure, seal it completely airtight, and move it into a vacuum, the vapor pressure above the water will still be at whatever it was before. No mass is entering or leaving the system. Now if you open that container in the vacuum, you've basically increased V at constant T, meaning that P will drastically go down according to Boyle's law. Vapor pressure above the water will correspondingly drop to near 0. Then, at room temperature, it would be much easier for the water to boil, as the "atmospheric pressure" is close to 0.

 

[betterfuture]

Okay so that means since the atmospheric pressure is close to 0, the boiling point would have to decrease because boiling point is determined by the atmospheric pressure. If atmospheric pressure goes way down, boiling point would have to go way down? Right?

So let me try to understand a little because it seems a little confusing. If you take a kettle of water up to an area of lower atmospheric pressure, the boiling point of water lowers as well. And this directly deals with the vapor pressure of the water. Vapor pressure is the pressure directly above the water that is made by the water evaporating and condensing in a closed container, right? So if boiling point is lowered, that would mean that the vapor pressure is higher in the container meaning you would only require a little heat to bring the water to its boiling point. And that is why boiling point lowers at low atmospheric pressure?

This is from what I understood about boiling point. I don't know if I am 100% correct on this.

 

[aldol16]

Okay so that means since the atmospheric pressure is close to 0, the boiling point would have to decrease because boiling point is determined by the atmospheric pressure. If atmospheric pressure goes way down, boiling point would have to go way down? Right?

Yes! The water would then boil sooner! This is actually the exact opposite of how a pressure cooker works.

So let me try to understand a little because it seems a little confusing. If you take a kettle of water up to an area of lower atmospheric pressure, the boiling point of water lowers as well. And this directly deals with the vapor pressure of the water. Vapor pressure is the pressure directly above the water that is made by the water evaporating and condensing in a closed container, right? So if boiling point is lowered, that would mean that the vapor pressure is higher in the container meaning you would only require a little heat to bring the water to its boiling point. And that is why boiling point lowers at low atmospheric pressure?

Almost. The vapor pressure only depends on temperature. So, assuming that the temperature change when you go up in elevation is negligible (imagine you're in a temperature-controlled RV or something), the vapor pressure will not change. However, the atmospheric pressure is lower now. And remember that boiling point is defined as the point at which vapor pressure equals atmospheric pressure. So the vapor pressure is now closer to atmospheric pressure and less heat is required to raise it to atmospheric pressure. Let's just make up numbers to illustrate. The vapor pressure at a given temperature is 100. The atmospheric pressure is 200. You bring it up a mountain in a temperature-controlled RV. The vapor pressure is still 100. But now the atmospheric pressure is 150. So instead of needing to bring the vapor pressure up by 100 to boil the liquid, you only need to bring the vapor pressure up by 50 - ergo, you require less heat.

 

[betterfuture]

Okay so basically to sum it up, vapor pressure DOES NOT change. Only the heat required to bring a vapor pressure to its boiling point changes because the environment's atmospheric pressure changed - from 200 to 150 in your given example.

 

[aldol16]

Okay so basically to sum it up, vapor pressure DOES NOT change. Only the heat required to bring a vapor pressure to its boiling point changes because the environment's atmospheric pressure changed - from 200 to 150 in your given example.

Yes. Vapor pressure depends only on temperature.

 

[betterfuture]

Thanks for your help!