Changes of State
When a substance is heated
or cooled, it can change from
one state to another.
Thermal Energy
Energy is the ability to
cause changes. Energy
of motion is called
kinetic energy.
All molecules are in
constant motion. The
total kinetic energy of
all the molecules in a
sample of a substance
is its thermal energy.
Which has the most
thermal energy, a glass
of ice water or the same
amount of hot coffee?
Answer: Hot coffee.
The molecules of coffee
are moving faster than the
molecules of ice water.
Which has the most
thermal energy, a glass
of warm milk or a lake
filled with cold water?
Answer: The cold lake.
Even though the molecules
in the lake are moving slower,
there are many many more
of them, so that means more
thermal energy.
Temperature is a measure
of the average kinetic energy
of the molecules of a substance.
Heat is the transfer of
thermal energy from
a substance with a higher
temperature to another
substance with a lower
temperature.
Which liquid has the higher
temperature? If you placed the
two glasses together so they 
touch, in which direction will
heat flow?
Answers: The hot
tea has the highest
temperature. Heat will
flow from the hot tea
to the ice tea.
Changing State
Melting is a change of
state from solid to liquid.
Crystalline solids melt at a
certain temperature called
the melting point. The melting
point of ice is 0 C.

Amorphous solids do not
have a melting point. They
just keep getting softer as
the temperature rises, like
glass.
Glass is an amorphous
solid. As it is heated it
gets softer and softer. 
Glass has no melting point.
melting
freezing
When heat is removed from
a liquid its molecules start
moving more slowly until it
becomes a solid. This is 
called freezing.
Freezing is the opposite
of melting. The freezing
point of a liquid is the
same as the melting point
of its solid. The freezing
point of water is 0 C.
During freezing or melting
the temperature of the
substance does not change.
vaporization
&
condensation
Vaporization is changing state
from a liquid to a gas. There
are two types of vaporization:
Evaporation
Boiling
Boiling happens when a liquid
changes to a gas beneath the
surface of the liquid. As a liquid
increases in temperature, its
molecules move faster until they
form gas bubbles under the surface.
Boiling takes place at
a certain temperature
called the boiling point.
For water, the boiling
point is 100 C.
Evaporation is changing
from liquid to gas at the
surface of a liquid. It can
happen at any temperature.
It explains why mud puddles
dry up.
Condensation is the opposite
of vaporization. It is changing
from gas to a liquid. When a
gas cools down or touches a
cool surface, it changes to a
liquid. This explains dew or 
why the outside of a cold glass
of water gets wet.
Some solids change directly
to a gas without going through
the liquid state. This is called
sublimation. Dry ice, which is
solid carbon dioxide, turns
directly back to a gas. The
opposite of sublimation is 
crystalization, where a gas is
turned directly to a solid, such
as in the formation of snowflakes.
Dry Ice causes water to immediately freeze.
Let's Review
Snowflakes are 6-sided
crystals caused by water
vapor turning into a solid.