vapor pressure and atm question

[orangetea]

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Which of the following statements is true?

a. standing on a lake that is frozen at 0 Celsius will cause some of the ice under your foot to melt
b. reducing the atmospheric pressure over a solution of water will increase that solution's vapor pressure.
c. who cares
d. who cares


So the answer is A.

and I picked B because I thought if you reduce atmospheric pressure you reduce boiling point which increases vapor pressure..

Why am I wrong 🙁

 

[Gnyc123]

i always thought so too

 

[Gnyc123]

nvm

 

[Dipraman]

Which of the following statements is true?

a. standing on a lake that is frozen at 0 Celsius will cause some of the ice under your foot to melt
b. reducing the atmospheric pressure over a solution of water will increase that solution's vapor pressure.
c. who cares
d. who cares


So the answer is A.

and I picked B because I thought if you reduce atmospheric pressure you reduce boiling point which increases vapor pressure..

Why am I wrong 🙁

Decreasing the atmospheric pressure (atm) decreases the vapor pressure (vp) which increases the boiling point. If we decrease atm then then the vapor molecules are not compacted as they were before hence the vapor pressure will be lower. Lower vp means that you need more energy to attain a vp at which the solution will boil hence higher boiling point.

Hope this helps

 

[Czarcasm]

Choice A is a unique feature of water you should recognize immediately. Usually liquids become solid when we increase pressure because solids are generally denser than liquids. Water is unique in that the liquid form is denser (hence why ice floats); by increasing the pressure, we shift to the more dense form, which in this instance, favors the formation of liquid.

Choice B, the key fact you should recall to eliminate this answer choice is that a liquid boils when its vapor pressure equals atmospheric pressure. Therefore, it should make sense then if we decrease atmospheric pressure, it's considerably easier for something to boil. It would require less vapor pressure. Unfortunately, as a consequence, the liquid requires less heat input and therefore, is less hot. Makes coffee less enjoyable 🙂

 

[orangetea]

Choice A is a unique feature of water you should recognize immediately. Usually liquids become solid when we increase pressure because solids are generally denser than liquids. Water is unique in that the liquid form is denser (hence why ice floats); by increasing the pressure, we shift to the more dense form, which in this instance, favors the formation of liquid.

Choice B, the key fact you should recall to eliminate this answer choice is that a liquid boils when its vapor pressure equals atmospheric pressure. Therefore, it should make sense then if we decrease atmospheric pressure, it's considerably easier for something to boil. It would require less vapor pressure. Unfortunately, as a consequence, the liquid requires less heat input and therefore, is less hot. Makes coffee less enjoyable 🙂

I thought vapor pressure was the amount of pressure the vapor exerted once it was boiling. I don't quit understand what you mean by "required" =/

I think that this question is more so asking about colligative properties and the concept that vapor pressure only depends on temperature and intermolecular forces.

 

[Czarcasm]

I thought vapor pressure was the amount of pressure the vapor exerted once it was boiling. I don't quit understand what you mean by "required" =/

I think that this question is more so asking about colligative properties and the concept that vapor pressure only depends on temperature and intermolecular forces.

Vapor pressure is just a property of solutions. If you ever been down south, you know how humid it can be. That's essentially the vapor in the air. But yet, the surrounding water isn't boiling. At any given instance, water molecules break free to vapor molecules. This process is in equilibrium, so at any given instant, you have a certain vapor pressure. As you add more heat, the vapor pressure will rise until it reaches a vapor pressure equal to the surrounding atmosphere, at which point the entire solution will begin to boil.

Consider what happens when you're at higher elevations (pressure decreases). Being at higher elevations, if you had a cup of water set on a table, some water molecules will break free (ie. it has some vapor pressure). If you add heat, what you'll find is, is that it would boil relatively fast simply because less vapor pressure is needed in this instance to reach a point at which vapor pressure = atmospheric pressure. That's essentially what I mean by 'requires less vapor pressure.'

At sea level, normal boiling point is when vapor pressure equals 1 atm.
At some higher elevation, it could instead be 0.3 atm.

 

[sillyjoe]

Which of the following statements is true?

a. standing on a lake that is frozen at 0 Celsius will cause some of the ice under your foot to melt
b. reducing the atmospheric pressure over a solution of water will increase that solution's vapor pressure.
c. who cares
d. who cares


So the answer is A.

and I picked B because I thought if you reduce atmospheric pressure you reduce boiling point which increases vapor pressure..

Why am I wrong 🙁

It is easiest to think about it the way @Czarcasm and @oldnaish described. Vapor pressure is an intrinsic property of the liquid that can only be adjusted by the temperature.

Whether a molecule is in the liquid or gas phase is dependent on the environment it is in. Lets say Compound X has a vapor pressure of 150 torr at 300 kelvin and that we put compound X into a chamber that is at 760 torr (1 atm) at constant temperature. We would assume that Compound X is in the liquid phase since Pressuresurroundings > Vapor pressure of Compound X (760 > 150)

However, if we lower the pressure to 130 torr we would expect Compound X to be present as a gas. Pressuresurroundings < Vapor pressure of Compound X. The vapor pressure is still 150 since the temperature is constant, but it is now greater than the surroundings so it changes phase.

Alternatively, lets say we put Compound X into a chamber allowing for a constant pressure of 300 torr. We would expect Compound X to have a vapor pressure of 150 and therefore be a liquid. However, if he heat Compound X up we can increase Compound X's vapor pressure to exceed the 300 torr of the chamber allowing for Compound X to vaporize into a gas.