Changes in State - Basic Concept:
Adding heat to a substance causes its particles to move faster. If there is
enough energy they will break the attractions holding them in that state
and move up to the next state.
Removing heat from a substance causes its particles to slow down and
allows the attractions to begin to affect the particles drawing them together
into a lower state.
During a phase change the temperature remains constant, because the
energy changes are associated with making/breaking attractions, not with
particle movements
Heating curve (Slides 3)
In a solid the particles are vibrating in place as energy is added the
particles move faster over a greater distance.
When the particles have gained enough energy they will break free of some
of the attractions between them and the other particles allowing them to
move more freely through out the substance. (Slide 4)
During the change in state the temperature remains constant. This is
because the energy is being used to break particle attractions. (Slide 5)
Melting - Change from a solid to a liquid
Freezing - Change from a liquid to a solid
Freezing and Melting occur at the same temperature, which one occurs
depends on whether heat is being gained or lost.
Adding heat to a liquid causes the particles to move faster.
Once they have gained enough energy they may break free of the
remaining attractions and escape from the liquid.
The process by which a liquid becomes a gas is called vaporization. (Slide 6)
The process by which a gas becomes a liquid is called condensation. (Slide 7)
While vaporization and condensation are taking place there is no change in
temperature because the energy changes are associated with changes to
particle attractions. (Slide 8)
There are 2 mechanisms by which vaporization can take place:
1) Boiling - a rapid change in state from a liquid to a gas that occurs
at a constant temperature.
2) Evaporation - a gradual change from a liquid to a gas that occurs
over a wide range of temperature.
Evaporation occurs because some of the particles in a sample have
enough energy to escape. (Slide 9)
Some materials can undergo a change in state between the solid state and
gaseous state without becoming a liquid. (Slide 10)
Sublimation - direct change from a solid to a gas without becoming a
liquid.
Examples: PDB, Dry Ice, Iodine
Deposition - Direct change from a gas to a solid without becoming a liquid.
Example: Frost