Phase Changes
• Physical change of matter from one phase to
another due to a transfer of energy.
Evaporation
• 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).
Condensation
• 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.
Evaporation-Condensation Rates
• Depends upon environment, if moist:
– Condensation > evaporation (warming)
– Evaporation > condensation (cooling)
• Equilibrium – state of balance.
Evaporation and condensation occurring at
equal rates.
Boiling
• 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.
Freezing
• 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.
Melting
• 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.
Sublimation
• Solid - gas.
• Example: mothballs that “evaporate”
without leaving a liquid, and when snow
on the ground “evaporates” skipping the
liquid phase altogether.
Phase Change Graph
Phase Change Diagram
Temperature °C
Heat of vaporization
C.P
100
Heat of fusion
Condenses
Boiling
Freezing
F.P
O
M.P
Melting
Heat Energy
B.P
Phase Changes
Phase Changes
From what
Phase-to-Phase
does change
occur
Absorb or
Release Heat
Energy?
Cooling or
Warming Process
Melting
solid-liquid
Absorb
Cooling
Freezing
liquid-solid
Release
Warming
Vaporization
liquid-gas
Absorb
Cooling
Condensation
gas-liquid
Release
Warming
Sublimation
solid-gas
Absorb
Cooling
Heat of Fusion
• 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?
800
Heat of Vaporization
• 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?
5400
Sample Problem
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.
745 calories