Section 3.3 – Phases
Changes
Key Concepts
(you should be able to answer these questions)
• What are six common phase changes?
• What happens to a substance’s
temperature and a system’s energy during
a phase change?
• How does the arrangement of water
molecules change during melting and
freezing?
• How are evaporation and boiling different?
Important Vocabulary
(leave room in your notes for definitions!)
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Phase change
Endothermic
Exothermic
Heat of fusion
Vaporization
Heat of vaporization
Evaporation
Vapor pressure
Condensation
Sublimation
Deposition
Phase Changes
• When more than one state of matter is present
in a system, scientists say there are two different
phases of matter
– Example: an iceberg floating in the ocean
• A phase change is a reversible physical change
that occurs when a substance goes from one
form of matter to another
• Melting, freezing, vaporization, condensation,
sublimation and deposition are six common
phase changes
Phase Changes - continued
• All phase changes share certain
characteristics related to energy and
temperature
• The temperature of a substance does not
change during a phase change
– This means one way we can recognize a
phase change is by measuring the
temperature of a substance as it is heated or
cooled
Phase Change Diagram for Water
What happens during phase
changes?
• Energy gets transferred between a
substance and its surroundings
• Energy is either absorbed or released
during a phase change
• In an endothermic reaction, the
substance absorbs energy from its
surroundings
• In an exothermic reaction, the substance
releases energy into its surroundings
Endothermic/Exothermic Reactions
• An example of an endothermic change is ice melting
– Where does the energy for melting come from?
• 1 gram of ice absorbs 334 Joules [J] of energy as it
melts
• This amount of energy is the heat of fusion, or how
much energy is needed to be absorbed for 1 gram of the
substance to melt, for water.
• Note that the heat of fusion varies from substance to
substance.
• What do you think happens when ice freezes?
• Application: Preventing crop freezing
Melting
• The arrangement of molecules in water
becomes less orderly as water melts and more
orderly as water freezes.
• Melting is change of a solid into a liquid. Since
energy is absorbed, this change is endothermic.
• For ice, molecules gain energy, which means
they vibrate more quickly until they gain enough
energy that they are able to overcome the forces
of attraction from the other molecules and move
from their fixed positions.
Freezing
• Freezing is the change of a liquid into a solid.
Since energy is released, the change is
exothermic.
• As the average kinetic energy decreases
because it is being released into the system, the
molecules of a substance move more slowly
until the forces of attraction start to have an
effect and pull them into an orderly arrangement.
• Often you think of freezing happening at cold
temperatures, but things freeze when they turn
from a liquid into a solid, so they can freeze at
really high temperatures.
Vaporization
• Vaporization is the change of a liquid into a gas.
Since a substance absorbs energy, it is
endothermic.
• 1 gram of water needs 2261 Joules of energy in
order to vaporize.
– This is the heat of vaporization, or how much energy
is required for one gram of a substance to change
from a liquid to a gas, for water.
• The heat of vaporization changes from
substance to substance.
Two types of vaporization
• The two types of vaporization are
evaporation and boiling.
• Evaporation occurs when some of the
molecules near the surface of a liquid are
moving fast enough to overcome the
forces of attraction from other molecules.
– A larger surface area means something can
evaporate faster.
• Examples of evaporation?
Evaporation
• In a closed container, water evaporates
into water vapor and collects above the
liquid.
• Vapor pressure is the pressure caused by
the collisions of the vapor with the walls of
the container.
• If the temperature of the vapor increases,
the vapor pressure increases as well.
Boiling
• Boiling occurs when the vapor pressure of a system is
equal to the atmospheric pressure.
• The temperature at which this happens is the boiling
point.
• As you increase the temperature of something you are
trying to boil, you increase the vapor pressure.
• When you boil a pot of water, the molecules move faster
and faster until the molecules below the surface of the
water have enough kinetic energy to overcome the
forces of attraction from neighboring molecules.
• Since vapor is less dense than the liquid, it rises to the
surface and the bubbles burst, releasing the vapor into
the air.
Boiling continued
• Note that the boiling point of something
depends on atmospheric pressure. At
standard pressure, the boiling point of
water is 100°C.
• The higher your altitude, the lower
atmospheric pressure is, which means
water will boil at a lower temperature.
Condensation
• Condensation is the change from a gas
into a liquid. Since this transfers heat into
a system from the substance, it is
exothermic.
• Examples?
Vaporization / Condensation
Application
• Refrigeration!
Sublimation and Deposition
• Sublimation is the phase change from a
solid to a gas/vapor without becoming a
liquid first. Since this change absorbs
energy, it is endothermic.
• Deposition is the phase change from a
gas/vapor into a solid without becoming a
liquid first. Since this change releases
energy into a system, it is exothermic.
• Examples?
Exit Ticket
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What are the 6 types of phase changes?
Which ones are exothermic?
Which ones are endothermic?
Provide an example of each.