Changes of State
Chapter 3 Section 3
Energy & Change of State
• The change of a substance from one
physical form to another is a change of state
or phase.
• Remember, in a physical change the identity
of the substance does not change, only it’s
physical form.
• Ice, liquid water, and steam are all made of
the same molecules.
• These substances are different only in the
arrangement and motion of the molecules.
Movement of Particles
• In different states, or phases, the molecules
move differently because they have
different amounts of energy.
• To change a substance from one state of
matter to another you must add or remove
energy.
• The five phase changes are melting,
freezing, condensation, vaporization, and
sublimation.
Melting
• When heat is added to a substance to cause
it to go from a solid to a liquid, melting has
occurred.
• Adding energy increases the temperature of
the solid, causing the molecules to move
faster.
• Melting will occur when a certain
temperature has been reached, the melting
point.
Melting
• Look at figure 2 page 75. Why wouldn’t
gallium, a metal element, be useful as
jewelry?
• Gallium melts at 30°C. Normal human body temperature
is 37°C, so gallium will melt in your hand.
• Melting occurs when molecules move fast
enough to overcome their attractions to each
other.
• Melting is endothermic because energy is
added.
Freezing
• When heat is removed from a substance
and it changes from a liquid to a solid,
freezing has occurred.
• The temperature at which a liquid changes
to a solid is the freezing point.
• Freezing and melting will occur at the
same temperature.
Freezing
• Look at figure 3 page 75. At a certain
temperature, what will determine whether water
will melt or freeze?
• The addition or removal of energy.
• For a liquid to freeze the attractions between the
molecules most overcome their motion.
• Removing energy will cause molecules to lock
into place.
• This is exothermic because energy is removed.
Vaporization
• A term not used in your textbook is
vaporization.
• This is the phase change from liquid to gas.
• There are two types of vaporization:
– Evaporation
– Boiling
• Vaporization requires the addition of heat,
so it is endothermic.
Evaporation
• When liquid changes to a gas at
temperatures below the boiling point,
evaporation has occurred.
• Evaporation occurs at the surface of the
liquid.
• Sweating helps cool your body because the
water evaporates from your skin, taking the
heat with it.
Boiling
• When liquid changes to a gas throughout
the liquid, not just at the surface, boiling is
taking place.
• The temperature at which a substance boils
is the boiling point.
• Pressure will effect the boiling point of a
liquid. When pressure is less, so is the b.p.
Condensation
• The change of state from a gas to a liquid
is condensation.
• The condensation point is the temperature
at which the gas becomes a liquid and is
the same as the boiling point of the
substance (at a given pressure).
• Energy must be removed for condensation
to occur, so it is exothermic.
Sublimation
• The change of state from a gas to a solid or
a solid to a gas (where the liquid phase is
bypassed) is sublimation.
• Sublimation can be endothermic or
exothermic.
• Dry ice, iodine crystals, and ice can all
sublime.
Change of Temp vs. Change of
State
• When most substances lose or gain energy,
one of two things will happen to the
substance (not both):
– The substance will change temperature
OR
– The substance will change state
Change of Temp vs. Change of
State
• When temperature changes, the speed of
the molecules changes.
• Temperature cannot change until the
change of state is complete.
• The temperature of boiling water stays at
100°C until all of the water has vaporized,
no matter how much more heat you add.
Figure 7 Page 79
• Look at Figure 7 on page 79. This is a
graph of the “Heat Curve” of water.
• At what temperature does water melt?
• At what temperature does water boil?
• Does the temperature change when the
water is changing states?
• Why? Why not?
Additional Questions
• Why is antifreeze added to automobile
engines during the summer as well as the
winter?
• What is the purpose of adding salt to
boiling water when making pasta, and
putting salt on icy roadways in the winter?
• Answer these questions by referring to
melting and boiling points.