Liquid water cools when it boils?

[Lunasly]

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This is passage 74 in TPRH science workbook (GC). It question asks: Based upon the thermodynamics of gases, which one of the following phenomena should not be real?

The author claims that "the heating of water by placing it I'm vacuum and allowing it to boil" is not a phenomenon that occurs. They claim that the remaining liquid water COOLS when it is placed in a vacuum and allowed to boil.

To my understanding, when you place water in a vacuum, the external pressure is reduced to a point where it equals the vapor pressure of the liquid water. Thus it is allowed to boil. Now, vaporization is an endothermic process. This means that heat must be supplied to the system in order for the water molecules to overcome intermolecular forces and evaporate. If the system requires heat, then why doesn't it heat up?

I've read before that endothermic chemical processes make the beaker cooler, while exothermic processes make it hotter.

I know that my misunderstanding of this concept is with regard to my perspective of the system and the surrounding.

Thanks for the help!

 

[sillyjoe]

What is the misunderstanding? I think you have it correct. In order to regularly boil water you increase the temperature of water and thus increasing the vapor pressure to exceed the atmospheric pressure. This makes sense since the gaseous state is the most energetic state with the highest kinetic energy. As a result it is an endothermic reaction where your stove supplies the heat/energy.

Alternatively you can boil water by lowering the pressure of the reaction vessel allowing water to boil since you can lower the pressure of the reaction vessel below the vapor pressure of water. Since vaporization is still an endothermic reaction the whole system will cool down since you are using ambient heat to vaporize.

 

[Lunasly]

Ah, ok. I suppose I did answer my own question. Since the surroundings is not supplying the heat, the reaction itself is endothermic thus it cools as it boils.