Liquids & Vapor Pressure

Vapor Pressure (VP) = Pressure exerted by vapor over its liquid How is vapor pressure affected by temperature? higher the temperature, the higher the VP

Vapor Gas phase of substance that is normally liquid at room temperature Evaporation occurs at all temperatures the easier a substance evaporates, the weaker the attractive forces between liquid molecules

open system Fig a: open system Evaporation continues until all liquid is gone closed system Fig b: closed system Rate of evaporation = rate of condensation System appears static due to equilibrium

all liquids: VP ↑as T ↑

Vapor Pressure VP does NOT depend on how much liquid is present VP depends ONLY on temperature

Phase Changes 1.Melting s → l 2.Freezing l → s 3.Vaporization (Boiling) l → g - occurs throughout liquid, temp stays constant Evaporation - occurs at surface; temperature drops 4. Condensation g → l 5. Deposition g → s 6. Sublimation s → g

Phase Changes are accompanied by energy changes Which phase changes are endothermioc (absorb energy)? Which phase changes are exothermic (release energy)? Melting, Vaporization, Sublimation FreezingCondensation, Deposition Freezing, Condensation, Deposition

Boiling Point Temperature at which: VP of liquid = external (atmospheric) P Normal Boiling PointNormal Boiling Point: temp at which VP of liquid = 1 atm (STP) normal bp water = 100˚C

Melting Point = temp at which liquid and solid phases of substance coexist at equilibrium normal mp water = 0˚C

Freezing Point Temp at which liquid is converted to crystalline solid How does freezing point compare to melting point? They’re the same: for water = 0˚C

Boiling and Pressure If ↑ external pressure (say are camping in Death Valley), the boiling point of water is ________ than 100 o C If ↓ external pressure (say are eating Raman noodles at top of Mt. Whitney), the boiling point of water is ________ than 100 o C > <

Boiling vs. Evaporation BOILING Occurs throughout liquid Occurs at definite temperature Occurs at constant temperature Spontaneous only when VP equals external pressure EVAPORATION Occurs at surface of liquid (or solid) Occurs at all temperatures Liquid cools off as it evaporates Spontaneous all the time