Topic: Liquids & Vapor Pressure Do Now: Compare I 2 and F 2 Why is F 2 a gas at room temp. and I 2 a solid Click here and go over slides from Wisc-Online before talking notes!

Vapor Vapor is the gas phase of substance that is normally liquid at room temperature Some evaporation occurs at all temperatures (like you saw with the ice cube) The easier a substance evaporates, the weaker the IMF

open system Fig a: open system Evaporation continues until all liquid is gone closed system Fig b: closed system System appears unchanged b/c it’s at equilibrium meaning Rate of evaporation = rate of condensation Copy this only

escape molecules in liquid phase are moving at slightly different speeds…some have enough KE (move fast enough) to “escape” from liquid phase – no extra energy (heat) needed How does evaporation occur? JUST FYI: NO NEED TO COPY THIS

Vapor Pressure –Pressure exerted by vapor over its liquid –VP is affected by temperature: higher the T, higher the VP

Vapor Pressure VP does NOT depend on how much liquid is present VP depends only on temperature as T ↑ the VP ↑ (because more of the liquid is in the gas phase)

Boiling Point

Temperature at which: VP liquid = external atmospheric P Normal Boiling PointNormal Boiling Point: temp at which VP liquid = 1 atm Substances with high bp’s have strong IMF’s Boiling Point

Boiling vs. Evaporation BOILING occurs throughout liquid occurs at definite T occurs at constant T only spontaneous when VP = external pressure EVAPORATION occurs at surface of liquid (or solid) occurs at all T liquid cools off as evaporates spontaneous all the time

Boiling and Pressure If increase external pressure (ex: camping in Death Valley), boiling point is ____ than 100 o C If decrease external pressure (ex: eating Raman noodles at top of Mt. Whitney), the boiling point is ____ than 100 o C > <

IMF in Liquids WEAK FORCES (more liquid turning into gas) High VP High rate evaporation Low bp STRONG FORCES (more liquid less gas) Low VP Low rate evaporation High bp

Strong or Weak attractive forces? High vapor pressure High boiling point Low boiling point Low vapor pressure Evaporates rapidly Evaporates slowly Weak Strong Weak Strong Weak Strong