Relationship Between Vapor pressure, Atmospheric pressure and Boiling point

[Ame535]

Hi,
I need help understanding their relationship.
From my understanding, the higher the vapor pressure, the lower the boiling point (inversely related)

However, at high altitudes, the vapor pressure is decreased because the atmospheric pressure is decreased, yet the boiling point decreases too? Here it seems that the vapor pressure and boiling point are proportionally related! I don't understand this.

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[FeralisExtremum]

At higher altitudes, the atmospheric pressure is decreased. The vapor pressure is not decreased - it is not affected by atmospheric pressure. Rather, what changes is the vapor pressure at which the substance will boil. Since a substance reaches its boiling point when vapor pressure = atmospheric pressure, it will boil at a lower temperature because a lower vapor pressure is needed for equilibrium to be reached with the lower atmospheric pressure.

 

[DAT Destroyer]

Hi,
I need help understanding their relationship.
From my understanding, the higher the vapor pressure, the lower the boiling point (inversely related)

However, at high altitudes, the vapor pressure is decreased because the atmospheric pressure is decreased, yet the boiling point decreases too? Here it seems that the vapor pressure and boiling point are proportionally related! I don't understand this.

Hello Ame535!

As elevation increases, Atmospheric pressure decreases because air is less dense at higher altitudes. Because the atmospheric pressure is lower, the vapor pressure of the liquid needs to be lower to reach boiling point. Therefore, less heat is required to make the vapor pressure equal to the atmospheric pressure. The boiling point is lower at higher altitude.

You need to remember a key point......a liquid boils when the vapor pressure equals the EXTERNAL PRESSURE..........if this drops......the Boiling point drops. It is true as you say......BP and VP vary indirectly.......that holds true when the EXTERNAL PRESSURE is not changed. .

Hope this helps..

 

[Ame535]

Thank you guys, this makes a lot of sense!

 

[Hopeful7419]

At higher altitudes, the atmospheric pressure is decreased. The vapor pressure is not decreased - it is not affected by atmospheric pressure. Rather, what changes is the vapor pressure at which the substance will boil. Since a substance reaches its boiling point when vapor pressure = atmospheric pressure, it will boil at a lower temperature because a lower vapor pressure is needed for equilibrium to be reached with the lower atmospheric pressure.

So i'm not sure if I should create a new thread, but does this mean that the since it will boil at the lower temperature, that the boiling point is lower? I thought that vapor pressure and boiling point at inversely proportional, but I'm confused about the relationship with the between low vapor pressure and high boiling point.

 

[FeralisExtremum]

So i'm not sure if I should create a new thread, but does this mean that the since it will boil at the lower temperature, that the boiling point is lower? I thought that vapor pressure and boiling point at inversely proportional, but I'm confused about the relationship with the between low vapor pressure and high boiling point.

1. Yes, the boiling point is lower.

2. Vapor pressure and boiling point are inversely proportional at constant external pressure. The hypothetical situation above involves a change in the external pressure that results in a lower vapor pressure for boiling - separate concept.