Why Liquid Boils: Explaining Saturated Vapor Pressure

In summary: 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. In summary, boiling occurs when the pressure in the gas bubbles equals the pressure of the surroundings.
  • #1
netheril96
194
0
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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  • #2


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.
 
  • #3


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?
 
  • #4


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.
 
  • #5


This thread is over two years old.
 

1. Why does liquid boil?

Liquid boils because the molecules in the liquid gain enough energy to overcome the intermolecular forces holding them together, causing them to escape into the gas phase.

2. What is saturated vapor pressure?

Saturated vapor pressure is the pressure exerted by the vapor molecules in equilibrium with a liquid at a given temperature. It is the maximum pressure that can exist before the liquid starts to boil.

3. How does temperature affect saturated vapor pressure?

As the temperature of a liquid increases, the average kinetic energy of the molecules also increases. This results in more molecules having enough energy to escape into the gas phase, leading to an increase in saturated vapor pressure.

4. What factors affect the boiling point of a liquid?

The boiling point of a liquid is affected by atmospheric pressure, intermolecular forces, and the nature of the liquid's molecules. Higher atmospheric pressure and stronger intermolecular forces will result in a higher boiling point, while a lower atmospheric pressure and weaker intermolecular forces will result in a lower boiling point.

5. Can a liquid boil at any temperature?

No, a liquid can only boil at temperatures at or above its boiling point. At lower temperatures, the liquid will not have enough energy to overcome the intermolecular forces and enter the gas phase.

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