Vapor pressure of a liquid homework

[glasshut137]

[SOLVED] vapor pressure

Homework Statement

the vapor pressure of a liquid at 25 degrees celcius is 200 torr. If the pressure over the liquid is lowered to 190 torr at 25 degrees celsius the liquid will...

a)evaporate
b)boil
c)freeze

Homework Equations

The Attempt at a Solution

ok, so i think it would evaporate since the vapor pressure would decrease and therefore the decrease in pressure holding the liquid together would allow evaporation to take place. Can someone tell me if my reasoning is correct?

 

[chemisttree]

You are close but consider that evaporation is usually applied to liquids under isobaric conditions. What happens to a liquid when you subject it to a slight vacuum (or slightly lower)?

 

[Blueshift5]

Your reasoning is correct. The vapor pressure of a liquid is the pressure at which its vapor is in equilibrium with its liquid phase at a given temperature. When the external pressure is lowered to 190 torr, it is lower than the vapor pressure of the liquid (200 torr), which means that the vapor molecules have a higher energy and can escape from the liquid surface more easily. This leads to evaporation of the liquid. If the external pressure is further lowered, the liquid may eventually reach its boiling point and start boiling, but in this case, it would simply evaporate. Freezing would not occur as it requires a decrease in temperature, not pressure.