Why Liquid Boils: Explaining Saturated Vapor Pressure

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Discussion Overview

The discussion centers on understanding the phenomenon of boiling in liquids, specifically focusing on the concept of saturated vapor pressure and the conditions under which boiling occurs. Participants explore the relationship between vapor pressure, surrounding pressure, and the effects of liquid density and gravity on boiling.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants seek a deeper understanding of why boiling occurs, noting that many resources provide limited explanations.
  • One participant explains that boiling involves the formation of gas bubbles throughout the liquid, which must have a pressure equal to or greater than the surrounding pressure to prevent collapse.
  • A participant questions whether the pressure of the liquid due to gravity should be considered in the boiling process, suggesting that it may not be negligible in certain conditions.
  • Another participant proposes that boiling may occur when the saturated vapor pressure reaches 1.1 times the atmospheric pressure, considering the effects of pressure differences at various depths in the liquid.
  • One participant notes that as bubbles rise during boiling, their pressure decreases, leading to an increase in bubble size until they reach atmospheric pressure.

Areas of Agreement / Disagreement

Participants express differing views on the role of liquid pressure due to gravity and the specific conditions under which boiling occurs. There is no consensus on the exact pressure thresholds for boiling, indicating ongoing debate.

Contextual Notes

Participants highlight the complexity of boiling, including factors such as liquid density, environmental pressure, and the influence of gravity, which remain unresolved in the discussion.

netheril96
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It's hard for me to find the why,bcz most books only give this without any explanation

Is there anyone who knows the exact reason?
 
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Boiling refers to the process whereby a phase change from liquid to gas takes place throughout the liquid. This would imply the formation of gas bubbles beneath the liquid surface. That is why bubbling is a characteristic of boiling liquids.

Now, the pressure in the gas bubbles must be at least equal to that of the surrounding fluid and the atmosphere/environment. Otherwise, the bubbles will collapse, and boiling does not occur. Thus, the boiling can only occur at the point where the vapour pressure (of the gas bubbles) equals the pressure of the surroundings.
 


Fightfish said:
Boiling refers to the process whereby a phase change from liquid to gas takes place throughout the liquid. This would imply the formation of gas bubbles beneath the liquid surface. That is why bubbling is a characteristic of boiling liquids.

Now, the pressure in the gas bubbles must be at least equal to that of the surrounding fluid and the atmosphere/environment. Otherwise, the bubbles will collapse, and boiling does not occur. Thus, the boiling can only occur at the point where the vapour pressure (of the gas bubbles) equals the pressure of the surroundings.

I read this sort of thing before.But why don't you calculate the pressure of the liquid due to gravity?Is it neglible?But if the liquid is so dense or the envrionmental pressure is so small that the bubbles some centimeters below the surface is under the pressure significantly larger,say 10%,than that of environmental pressure of the gas above the surface,will it boil when saturated vapor pressure reaches environmental pressure?Or when it reaches 1.1 times the environmental pressure?
 


I think you are right netheril96. It should boil when the pressure reaches 1.1 times the atmospheric pressure(neglecting the pressure difference due to surface tension of bubble).
I think that is why the bubble gets larger as it rises up during boiling. The pressure in the bubble decreases with the height and it gets bigger and bigger, finally reaching to the atmospheric pressure.
 


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