[roadkillbunny]

Hello,
I am part of a AP course at my school and we have got a summer assignment. It is basiccaly to take notes on 2 chapters. On my reading I came upon a statement that says boiling point is when the vapor pressure of a aliquid equals the pressure acting on it's surface. The book goes on to explain why this is important but not why this is true. So I am wondering, why? I can't seem to figure it out.

Also I got one more question about it. If we have a liquid in a evaporated container, dosn't that mean there is no pressure acting on top of it? Thus it should boil right away.

[sdekivit]

You must overcome the external pressure done on the system (your aliquid) to evaporate. Compare with a piston where a gas is expanding. It needs to overcome the external pressure to move the piston. Now the molecules need to overcome the external pressure done on the aliqiud to evaporate.

--> the point where evaporation begins, thus wqhen the external pressure equals the pressure of the aliquid, is called the bioling point.

[Borek]

Boiling is evaporation taking place in volume, not only on the liquid surface. Thus vapor pressure must be the same as external pressure.

[xiankai]

i thought that vapor pressure was the pressure at which the liquid-gas conversion was in equilibrium? does it have to do something with raising the temperature? seeing that the change of state is endothermic, would the increase in temperature would favor the gas-phase and hence eventually evaporate all the liquid?

[camur]

vapor pressure is the pressure exerted by vapor which is at equilibrium with own liquid. when vapor pressure is equal to the atmospheric pressure boiling starts. consider a bubble at the bottom of the liquid. bubble moves up and wants to pass gas phase.for this operation the pressure in the bubble must be equal to external pressure
remember, boiling means the vaporization of liquid from each point of liquid

[Donaldson Tan]

i thought that vapor pressure was the pressure at which the liquid-gas conversion was in equilibrium? does it have to do something with raising the temperature? seeing that the change of state is endothermic, would the increase in temperature would favor the gas-phase and hence eventually evaporate all the liquid?

the liquid-gas equilibrium occurs at a specific sets of pressure and temperature. however, boiling point is only pressure-dependent. the higher the pressure, the more energy the liquid should contain such that its pressure equals that of the surroundings, such that it boils. This means one has to heat the liquid to raise its pressure, and consequently its temperature. When boiling occurs, this temperature is known as the boiling point.

At equilibrium, supplying heat favours the process that eliminates heat, ie. the endothermic process. In vapour-liquid equilibrium, conversion from liquid to gas is the endothermic process.

[JZ_1]

Temperature doesnt have much to do with boling except supply the ENERGY for the liquid to change to the gas phase. your liquid could also "boil" if u lower the atmospheric pressure....

think about this...
HERE is your pot of water..as u can see theree is only a few bubbles

The bubbles form at the bottom...for it to boil, it must reach the top of the water...(by definition of boiling).

   |                     |++++++++
   |/\/\/\/\/\/\/\/\|
   |                     |
   |       ?              |
   |______?_____?_|

IT wont reach the top...i'lll collapse before it reaches the top...    how sad
until it has enough energy to overcome the atmospheric pressure, i'tz not boiling...

the fire continually supplys the energy  

once it has enough..the bubbles will rize to the top...and that's boiling...


NOTE: water boils at 100° C at SEA LEVEL
         Water may boil at 98° or lower if you are higher up
(since there is less atmospheric pressure pushing down on the water (bubble), it wont collapse)

race boiling water..(one on mountain..one at sea level) the mountain will win..

Hope this helps.....hope it'll stick to your head

[Borek]

The bubbles form at the bottom...for it to boil, it must reach the top of the water...(by definition of boiling).

No.

the bubbles will rize to the top...and that's boiling...

No.

Boiling means evaporation goes not only on the surface of the liquid, but in the whole volume. However, that's the ideal case. Everything you describe are sideffects of the limited speed at which the heat is transferred, of the gravitation creating additional pressure below the liquid surface and of the overheating at the bottom of the pot.