Liquids

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Liquids Vapor Pressure
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Vapor

• Gas phase of substance that is normally liquid at
  room temperature
• Some evaporation occurs at all temperatures
• The easier a substance evaporates, the weaker the
  IMF

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Fig a open system Evaporation continues
until all liquid is gone Fig b closed system
Rate of evaporation rate of condensation
System appears static due to equilibrium
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How does evaporation occur?
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molecules in liquid phase have range of KE
some have enough KE to escape from liquid phase
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Vapor Pressure

• Pressure exerted by vapor over its liquid
• VP is affected by temperature
• higher the T, higher the VP

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all liquids VP ?as T ?
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Vapor Pressure

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

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IMF in Liquids

• WEAK FORCES
• High VP
• High rate evaporation
• Low bp
• Small Hv

• STRONG FORCES
• Low VP
• Low rate evaporation
• High bp
• Large Hv

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Phase Changes

• Melting
• Boiling
• Vaporization
• occurs throughout liquid T constant)
• Evaporation (occurs only at surface T ?)
• Sublimation
• Freezing
• Deposition
• Condensation

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Phase Changes are accompanied by energy changes

• Endothermic Process Energy is absorbed
• Exothermic Process Energy is released

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• Which phase changes absorb energy (endothermic)?
• Which phase changes release energy (exothermic)?

melting, boiling (vaporization), sublimation
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Boiling Point

• Temperature at which
• VP liquid external atmospheric P
• Normal Boiling Point
• temp at which VP liquid 1 atm
• Substances with high bps have strong IMFs

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Melting Point

• temp at which liquid solid phases of
  substance coexist at equilibrium
• higher the mp, the stronger the IMF

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Freezing Point

• Temp at which liquid is converted to crystalline
  solid
• How does fp compare to mp?

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Strong or Weak attractive forces?

• High vapor pressure
• Large Hv
• High boiling point
• Low boiling point
• Low vapor pressure
• Small Hv
• Evaporates rapidly
• Evaporates slowly

• Weak
• Strong
• Strong
• Weak
• Strong
• Weak
• Weak
• Strong

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Boiling and Pressure

• If increase external pressure (ex camping in
  Death Valley), boiling point is ____ than 100oC
• If decrease external pressure (ex eating Raman
  noodles at top of Mt. Whitney), the boiling point
  is ____ than 100oC

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Boiling vs. Evaporation

• EVAPORATION
• occurs at surface of liquid (or solid)
• occurs at all T
• liquid cools off as evaporates
• spontaneous all the time

• BOILING
• occurs throughout liquid
• occurs at definite T
• occurs at constant T
• only spontaneous when VP external pressure