Vapour Pressure and External Pressure

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Discussion Overview

The discussion revolves around the relationship between vapour pressure and external pressure, specifically exploring the behavior of liquids in a vacuum and the conditions under which they boil or evaporate. The scope includes theoretical considerations and conceptual clarifications regarding phase changes and the effects of external conditions on liquids.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that a liquid will boil when its vapour pressure equals the external pressure and questions whether a liquid in a vacuum will boil instantly when heated.
  • Another participant suggests that the boiling behavior depends on the liquid and temperature, indicating that many liquids may boil at room temperature in a vacuum.
  • A different viewpoint emphasizes that a vacuum has no temperature and argues that a liquid in a vacuum will not boil but may turn to vapor directly without boiling, suggesting that this process could occur spontaneously without added heat.
  • A participant raises a question about how to determine whether a substance will exist as a liquid or gas in a vacuum.
  • It is noted that vapour pressure is a function of temperature, and while a substance with positive vapour pressure will evaporate in a vacuum, the specifics of the situation, such as microgravity and temperature gradients, will influence the process.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of liquids in a vacuum, with no consensus reached on whether they will boil or evaporate under specific conditions. The discussion remains unresolved regarding the precise mechanisms and conditions affecting phase changes in a vacuum.

Contextual Notes

Participants highlight various factors that could influence the boiling or evaporation of liquids in a vacuum, including temperature, the presence of microgravity, and contact with other materials. These factors introduce complexity and uncertainty into the discussion.

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When the vapour pressure of a liquid equals to the external pressure, the liquid will boil.
So, what if a liquid is placed in a vacum? Will the liquid boil instantly when heated?
 
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It depends on the liquid and the temperature, but many will boil at room temperature in a vacuum.
 
Temperature is a measure of the average kinetic energy of molecules.

A Vacuum is devoid of all matter by definition and therefore has no temperature.

If your substance exists as a liquid state in a vacuum, then it will not boil; however, if it exists as a gas in vaccum, then it will boil. (Actually, boiling occurs because of microscopic bumps on the pot that agitate the liquid. It would probably turn to vapor directly without boiling in outer-space)

You won't have to add heat to achive this, it will occur spontaneously.
 
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How do we know whether a substance will exist in liquid or gas form in vaccum?
 
Vapor pressure of a liquid (or even a solid) is a function of temperature.
For sure, if it is positive the substance will evaporate when placed in vacuum.
This doesn't mean that this will happen like in a boiling pan.
The details will play a role: is it in microgravity of not, is the temperature uniform or is there a temperature gradient, is the substance in contact with some other material, ...
 

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