Study Guides
Big Picture
A phase diagram shows under which conditions a substance will be a solid, liquid, or gas. While we often think of
substances as having one single melting point temperature and one boiling point temperature, variations in pressure
alter these two temperatures. Phase diagrams are an easy way to understand the effects of temperature and pressure
on a substance.
Key Terms
Phase: A form or state of matter. The phase of a matter is dependent on the temperature and pressure.
Chemistry
Phase Diagrams
Phase Change: Change from one form of matter to another.
Melting (also called fusion): Change from solid to liquid. Particles gain enough potential energy to overcome the
intermolecular forces in a solid lattice and break free into a liquid state.
Freezing (also called solidification): Opposite of melting (change from liquid so solid).
Vaporization: Change from liquid to gas. Particles gain enough potential energy to overcome all intermolecular
forces and break free into the gas state.
Boiling: One type of vaporization is boiling, which occurs at a substance’s boiling point. All particles at boiling
point have enough energy to enter the gas phase.
Evaporation: Another type of vaporization, it can occur at any temperature in the liquid phase. Though not
all particles have enough energy to enter the gas phase, there will always be a few at the surface that will
evaporate into a gas. Eventually, given enough time, all the particles will become gas.
Condensation: Opposite of vaporization (change from gas to liquid). Particles lose the energy required to stay
apart and intermolecular forces pull the substance back into a liquid.
Sublimation: Particles get enough potential energy to go straight from the solid phase to the gas phase. Solid
carbon dioxide does this under normal conditions.
Deposition: Opposite of sublimation (change from gas to solid).
Vapor Pressure: The pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the
liquid (or solid) in a closed container.
Triple Point: Conditions in which a substance can exist as a solid, liquid, and a gas.
Critical Point: A certain temperature and pressure above which a supercritical liquid is formed. Here, the liquid
expands to fill the container and is in a state between liquid and gas.
Phase Changes
All matter can exist in different phases. As the temperature and/or pressure changes, matter can undergo a phase
change.
• In a phase change, only the physical form (not the chemical identity) of the substance changes.
• During a phase change, temperature does not change, but potential energy does.
• A phase change is a reversible physical change.
By...
Solid
Liquid
Gas
Melting
Sublimation
Liquid
Solid
Gas
Freezing
Vaporization
Gas
Solid
Liquid
Deposition
Condensation
Notes
This guide was created by Steven Lai, Rory Runser, and Jin Yu. To learn more
about the student authors, visit http://www.ck12.org/about/about-us/team/
interns.
Page 1 of 2
v1.1.12.2012
Disclaimer: this study guide was not created to replace
your textbook and is for classroom or individual use only.
Changes to...
Chemistry
Phase Diagrams
cont .
General Phase Diagram
A phase diagram shows the relationship between
pressure and temperature and the effects it has on
a substance. Normally, we live at a pressure of 1
atmosphere, so we only see a small fraction of this
relationship. In a phase diagram, we can see how
changing temperature and/or pressure affects the
phase of a substance.
• Increasing
temperature causes a substance to
go from solid to liquid to gas as the particles
gain enough kinetic energy to overcome the
intermolecular forces.
• Increasing
pressure causes a substance to go
from gas to liquid to solid as the molecules are
forced closer together.
• Liquids
and solids have a vapor pressure.
The vapor pressure is a result of particles
escaping to the gas phase. When the
vapor pressure is greater than the external
pressure, the number of particles escaping
from the liquid or solid is greater than the
number of gas particles returning to the
liquid or solid. Solids have lower vapor
pressures than liquids do.
• Sublimation happens when solids have vapor
Figure: A phase diagram. There are two green lines–one solid and one
dotted. Only one green line is present for any given phase diagram. For
most substances, the green solid line is part of the diagram. For water,
the green dotted line is present.
pressures greater than atmospheric pressure
at near room temperature, resulting in the
solid changing into a vapor without entering
the liquid state.
Image Credit: Matthieumarechal, GNU-FDL 1.2
Features of a phase diagram:
• Triple
point–a special set of conditions (pressure and temperature) that allows all three phases to exist in
equilibrium at one time
• The lines separating two phases are the conditions that allow the two phases to exist in equilibrium at the same time
• Critical
point–beyond this point, vapor (gas) and liquid cannot exist in equilibrium; increasing the pressure will
not keep the substance in liquid form
The phase diagram can be used to find the state of matter under certain conditions.
• Look at the graph. Use the pressure and temperature to pinpoint the location.
• Use the graph to determine which section the point is in. This lets you identify the phase.
Phase Diagram for Water
It is important to understand why the green dotted line, not the green solid line, is present in the phase diagram for
water.
Unlike most substances, which have a denser solid phase than liquid phase, the spread-out crystal lattice of solid water
(ice) causes it to be less dense than liquid water (this is why ice floats). The hydrogen bonds hold the water molecules
in the solid phase in an open framework that looks like a honeycomb. Thus, increasing the pressure on ice will cause
the crystal lattice to break down and form water instead.
As a result, the line between solid and liquid for water goes in the opposite direction of that of most substances.
Page 2 of 2