Chemistry Notes: Phases of
Matter
Chemistry 2014-2015
States of Matter and Phase Changes
 The three most common phases, or states
of matter, are solids, liquids, and gases.
 We can describe each state of matter in
several ways.
 Compressibility is a way of measuring
how little or how much a substance can
be compressed. The picture on the
right shows that a gas can be
compressed significantly, while a liquid
can only be compressed slightly.
 Structure: the arrangement of particles in a substance, and
their ability to move
 Relative kinetic energy of the particles in a substance (kinetic
energy = energy of motion)
 Shape (definite or not definite)
 Volume (definite or not definite)
 Particle diagrams are a way of representing the structure and
motion of particles in a substance. The picture on the right
shows particle diagrams for a solid, a liquid, and a gas.
Solids
 Very low compressibility
 Structure is generally very organized;
some solids have an orderly crystalline
structure, while others are more
amorphous. The particles within a
solid are stationary (they cannot
move from one place to another).
 Relatively low kinetic energy
compared to liquids and gases
 Definite shape and volume
Liquids
 Very low compressibility
 Liquids have a more chaotic structure
than solids, because their particles
can “slide” past each other.
 Liquids have higher kinetic energy
than solids, but less kinetic energy
than gases.
 Liquids have definite volume, but
their shape changes to fit the shape
of their container.
Gases
 High compressibility (can be compressed
and can expand)
 Gases are composed of particles that move
past each other, and collide with each
other, at high speeds.
 Gases have higher kinetic energy than
liquids or solids.
 Gases do not have definite shape or
volume.
Phase Changes
 A phase change is the transition of a substance from one
phase to another. Because phase changes only change
the phase of a substance and not its composition, they are
physical changes.
 Types of phase changes
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Boiling/vaporization/evaporation: transition from liquid to gas
Condensation: transition from gas to liquid
Melting: transition from solid to liquid
Freezing: transition from liquid to solid
Sublimation: transition from solid to gas (ex. dry ice, which is solid CO2)
Deposition: transition from gas to solid
 Phase changes can be classified as endothermic (if
they absorb or require energy) or exothermic (if they
release energy).
 Endothermic changes occur when a substance transitions
from a lower-energy state to a higher-energy state (boiling,
vaporization, evaporation, melting, sublimation).
 Exothermic changes occur when a substance transitions
from a higher-energy state to a lower-energy state
(condensation, freezing, deposition)