Chapter 2: Solids, Liquids, and
Gases
SECTION 2: CHANGES OF STATE
Changes Between Solid and Liquid
 Melting
 Solid to liquid
 Increase in thermal energy
 Point at which melting occurs is called melting point
 At melting point, particles of a solid are vibrating so fast that
they break free from their fixed position
 During melting, any energy continues to change the
arrangement of the particles so the temperature does not
increase
Melting
Checkpoint 1
 Describe what happens to ice when it reaches 0o C.
Checkpoint 1
 Describe what happens to ice when it reaches 0o C.
 The molecules of water vibrate until they can break
free from the crystal structure.
Changes Between Solid and Liquid
 Freezing
 Liquid to solid
 Reverse of melting
 Loss of energy
 Freezing point=melting point
 At freezing point, particles of a liquid move so slowly that they
begin to form a regular pattern
 During freezing, any loss of energy contributes to change in
arrangement of molecules so temperature does not decrease
Changes Between Solid and Liquid
Changes Between Liquid and Gas
 Vaporization
 Liquid to gas
 Takes place when particles in a liquid gain enough energy to
form a gas
 Two main types:
Evaporation: takes place only on the surface when particles gain
enough energy to escape into the air
 Boiling: when a liquid changes to a gas below its surface as well
as at the surface
 Boiling point: temperature at which liquid boils

• Affected by pressure: lower pressure
point
lower boiling
Vaporization
Checkpoint 2
 Both evaporation and boiling are changes from
liquid to gas. How are they different?
Checkpoint 2
 Both evaporation and boiling are changes from
liquid to gas. How are they different?
 Evaporation only takes place on the surface of a
liquid. Boiling takes place both on and below the
surface.
Changes Between Liquid and Gas
 Condensation
 Gas to liquid
 Opposite of vaporization
 Occurs when particles in a gas lose enough thermal energy to
form a liquid
 Ex. Formation of clouds and rain; “sweat” on glass on hot day;
fogging up a window with your breath
Condensation
Changes Between Solid and Gas
 Sublimation
 Solid to gas
 Particles of a solid do not pass through the liquid state as they
form a gas
 Ex. Dry ice (frozen carbon dioxide)
Particles Are Like People
 If you heat up a crowded room, people will start to
become agitated and move away from one another,
just like particles in a container
Summary Diagram
Checkpoint 3
 Which phase change goes directly from solid to gas?
Checkpoint 3
 Which phase change goes directly from solid to gas?
 Sublimation
Post-lesson Quiz
1. If a substance has a melting point of 50o C what is its
freezing point?
a. 100o C
b. 25o C
c. 75o C
d. 50o C
Post-lesson Quiz
2. Which phase change occurs when a gas becomes a
liquid?
a. condensation
b. melting
c. freezing
d. sublimation
Post-lesson Quiz
3. What happens to particles when heat is added?
a. they move faster and further apart
b. they move faster and closer together
c. they move slower and further apart
d. they move slower and closer together
Post-lesson Quiz
4. Which phase change occurs when a liquid becomes a
gas?
a. melting
b. freezing
c. vaporization
d. condensation
Post-lesson Quiz
5. What happens to boiling point when pressure
decreases?
a. It stays the same.
b. There is not enough information to know.
c. It increases.
d. It decreases.