Phase Changes


A PHASE CHANGE is a reversible
physical change that occurs when a
substance changes from one state
of matter to another
The temperature of a substance
does not change temperature
during a phase change.
Phase Changes
Phase Changes

Melting


Solid changes to liquid
Vaporization
(boiling)


Freezing

Liquid changes to solid

melting point = freezing point
 Sublimation


Solid changes to gas
EX: dry ice
Evaporation


liquid changes to gas
below the boiling
point
Condensation


liquid changes to gas
at the boiling point
Gas changes to liquid
Deposition


Gas changes to solid
Ex. frost
Phase Changes
Energy is absorbed or released during
a phase change.

Endothermic – absorbs energy from
surroundings (gets cold)


Endothermic reactions get cold because
energy is needed to form the products, which
are higher energy than the reactants.
Exothermic - releases energy to
surroundings(gets hot)

Exothermic reactions release heat because
the products are less energy than the
reactants. Heat accounts for this extra energy
and can be identified by a rising temperature.
Endo or Exo?


Which processes are exothermic?
Which processes are endothermic?
endothermic is adding heat
or energy
(ENdothermic heat is
ENtering)

Boiling




Vaporization


endothermic
Endothermic
Evaporation

endothermic
Freezing


endothermic
Melting


endothermic
Sublimation

exothermic is removing
heat or energy
(EXothermic heat is EXiting)
Condensation


exothermic
exothermic
Deposition

exothermic
Heating Curves

Kinetic Energy



motion of particles
related to temperature changes
Potential Energy


space between particles
related to phase changes
B. Heating Curves
Gas - KE 
Boiling - PE  (no temp
change)
Liquid - KE 
Melting - PE  (no temp
change)
Solid - KE 
Heating Curves

Heat of Fusion

energy required to change from solid to
liquid (melting)



One gram of ice absorbs 334 joules (J) of
energy as it melts
some attractive forces are broken
Usually energy is needed to break
bonds and energy is released when
bonds form.
Heating Curves

Heat of Vaporization




energy required (absorbed) to change
from liquid to gas
One gram of water gains 2258 joules of
energy when it vaporizes at 100° C
all attractive forces are broken
EX: steam burns, sweating