Change of Phase
Change of Phase
(Ch. 17)
(plasma), gas, liquid, solid…
solid…
PH 104 w/ dr. g
Lec 19
y Evaporation and Condensation
y Boiling
y Melting and Freezing
y Energy Transfers
How does it
“anti-freeze”?
Change of Phase
Change of Phase
Evaporation and Condensation
Evaporation and Condensation
y Evaporation: molecules escaping from liquid: liquid to gas
y Cools the liquid
y Rate increases with temperature
y Continues until vapor reaches
y
y
Closed container: equilibrates. Vapor pressure ~ temperature
Open to atmosphere: will dry up
y Condensation: molecules crashing into liquid:
y Warms the liquid
y More likely at lower temperatures
y
y
Clouds: from cooling of expanding, rising warm air
Fog: moist air over water moving to over cooler land (or water)
Change of Phase
Change of Phase
Boiling
Boiling
y Boiling = phase change: liq Ægas (vs evaporation: random)
y In interior of liquid.
y Pressure inside liquid: approximately =
y When vapor pressure > atmospheric,
atmospheric, bubbles can form
y
y
Occurs at specific temperature : BOILING POINT
y Lower temperature: bubbles cannot form (not enough pressure!)
LessLess-dense bubbles rise to surface, vapor escapes (with heat)
atmospheric
y Vapor pressure ~ temperature
y Higher atmospheric pressure:
y
(pressure cooker, geysers)
y Lower atmospheric pressure:
y
(in a vacuum: water can boil at the freezing point!)
atmospheric
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Change of Phase
Change of Phase
Melting and Freezing
Melting and Freezing
y Melting: solid to liquid
y heat absorption Æ more kinetic atoms
y Freezing: liquid to solid
y heat loss Æ less kinetic atoms
y Depends on:
y Temperature: freezing point : at atmospheric pressure
y Composition (substance, impurities)
y
Salts in water:
y Pressure: for most liquids, higher pressure induces freezing
y
WATER: higher pressure induces MELTING
Change of Phase
Energy Transfers
y Heating:
y Energy is ABSORBED by the solid or liquid
y raises temperature to next phase-change temperature
y causes the phase change (temperature constant during change)
y
Heat required =
y of fusion: water =
y of vaporization: water =
y (Cooling: same, but in reverse: energy RELEASED)
y In your lab: CALORIMETRY: predicts final temperature of
a mixture/process
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