evaporation/ boiling

[superduper12]

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for evaporation to occur:::

atmosphere pressure > vapor pressure > partial pressure above liquid



for boiling to occur:::

vapor pressure > atmosphere pressure ...where does partial pressure fit in...

I understand atmosphere pressure but could someone clarify the difference between partial and vapor pressure?

 

[justdoit31]

My thought is that vapor pressure is the total amount of pressure coming from the surface of the liquid as molecules go from the liquid to gas phase then condense back.

The partial pressure above the liquid is the vapor pressure of a specific molecule from the surface

For example if you have a salt water solution- some of the surface would have the dissociated Na+ or Cl- molecules and some would have H2O. The percentage of the H2O times total vapor pressure gives you the partial pressure which is attributed to the H2O- so if total pressure was 900 torr and 50% was water the partial pressure is 450 torr.

 

[superduper12]

vapor pressure is the equilibrium at which molecules leaving (escaping the intermolecular bonds of the solution) equals the molecules entering the solution.

vapor pressure increases with increasing temperature. a liquid boils when atmospheric pressure equals vapor pressure.

now this is what i don't get....the melting point is the temperature at which the vapor pressure of the solid is equal to the vapor pressure of the liquid. above the melting point, the liquid vapor pressure is greater than that of the solid. below the melting point, the liquid vapor pressure is less than solid. ...anyone understand this?