• 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