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Why liquid boils when the envrionmental pressure equals its saturated vapor pressure

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netheril96
#1
Feb24-10, 07:53 AM
P: 193
It's hard for me to find the why,bcz most books only give this without any explaination

Is there anyone who knows the exact reason?
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Fightfish
#2
Feb24-10, 09:56 AM
P: 623
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.
netheril96
#3
Feb24-10, 10:12 AM
P: 193
Quote Quote by Fightfish View Post
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?

manoj0071991
#4
Sep11-12, 11:41 PM
P: 2
Why liquid boils when the envrionmental pressure equals its saturated vapor 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.
Vanadium 50
#5
Sep12-12, 06:06 AM
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