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• Physical change of matter from one phase to another due to a transfer of energy.
Phase ChangesPhase Changes
• Liquid – gas at surface of a liquid.
• Molecules gain KE to become a vapor. Molecules left behind lose KE.
• Cooling process – molecules left behind lose KE (cooler).
EvaporationEvaporation
• Gas – liquid.
• Gas molecules ↓KE when collide with cold surface. Condense to liquid phase.
• Warming process – KE lost by condensing gas molecules warms the surface they strike.
CondensationCondensation
Evaporation-Condensation RatesEvaporation-Condensation Rates
• Depends upon environment, if moist:– Condensation > evaporation (warming)– Evaporation > condensation (cooling)
• Equilibrium – state of balance. Evaporation and condensation occurring at equal rates.
• Liquid – gas beneath the surface of a liquid.
• Bubbles of vapor form beneath the surface – rise – break free to the vapor phase.
• ↑atm. pressure - ↑boiling pt.
• Cooling process – the water is being cooled relative to the ↑temp it would attain otherwise. Because of cooling, it remains 100 °C instead of getting hotter.
BoilingBoiling
• Liquid - solid.
• Energy is extracted, molecules slow down and the molecular attraction overcomes the KE.
• Warming process – when you make ice cubes, you put liquid water in the freezer. The freezer cools the water, taking energy out – so it must give off energy.
FreezingFreezing
MeltingMelting
• Solid – liquid.
• Heat energy is added until KE is greater than the molecular attraction.
• Cooling process – the source of energy is the object or material around the stuff that is melting. The source cools as energy goes to the melting object.
• Solid - gas.
• Example: mothballs that “evaporate” without leaving a liquid, and when snow on the ground “evaporates” skipping the liquid phase altogether.
SublimationSublimation
Phase Change GraphPhase Change Graph
Te
mp
era
ture
°C
Phase Change DiagramPhase Change Diagram
Heat Energy
Freezing
Melting
Condenses
Boiling
M.P
F.P B.P
C.P
O
100 Heat of fusion
Heat of vaporization
Phase ChangesPhase Changes
Phase Changes
From what Phase-to-
Phase does change occur
Absorb or Release Heat
Energy?
Cooling or Warming Process
Melting
Freezing
Vaporization
Condensation
Sublimation
solid-liquid Absorb
liquid-solid Release
liquid-gas Absorb
gas-liquid Release
solid-gas Absorb
Cooling
Warming
Cooling
Warming
Cooling
• Amount of heat needed to change 1g substance from solid – liquid phase.
• 80 calories/g for water.
• Example: How many calories are needed to change 10g of ice at 0 °C to 10g of water at 0 °C?
Heat of FusionHeat of Fusion
800
• Amount of heat needed to change 1g substance from liquid – gas phase.
• 540 calories/g for water.
• Example: How many calories are needed to change 10g of water at 100 °C to entirely to water vapor?
Heat of VaporizationHeat of Vaporization
5400
How much heat is required to change 1g of ice at -20°C to water vapor at 130°C?
1.Q = mc∆t 1g · 0.5 cal/g-C · 20 °C = 10 cal.
2.Hf 1g · 80 cal/g = 80 cal.
3.Q = mc∆t 1g · 1 cal/g-C · 100 °C = 100 cal.
4.Hv 1g · 540 cal/g = 540 cal.
5.Q = mc∆t 1g. · 0.5 cal/g · 30 °C = 15 cal.
Sample ProblemSample Problem
745 calories
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