Figure 7.13 Definitions and energies of phase changes (a) The energy of a gas is much gr
eater than the energy of a liquid,
which is greater than the energy of a solid. Heat,
ΔH, must be
supplied to carry out a change from a lower energy state to a higher energy state.
ΔH
is the heat of fusion, fus
ΔH
vap
is the heat of
vaporization, and
ΔH
sub
is the heat of sublimation. (b) A molecular
view of the various changes in state.
evaporate
H
vapHD
subHD
fusHD
condense
sublime
deposit
gas liquid solid
(a)
(b)
freeze
melt
draw the liquid up the walls of the glass and produce the concave shape of the meniscus (Figure7.12a). Mercury atoms do not interact
well with glass, so the adhesive forces are
very weak. Consequently, mercury forms a c
onvex meniscus (Figure 7.12b) to maximize
Hg-Hg interactions and minimize interac
tions with the walls of the container.
7.6
CHANGES IN STATE
Molecules in a solid orient th
emselves to maximize their interactions, which lowers their
potential energy. Indeed, solids have the lowest
potential energy of the three states of
matter. Molecules in the liquid state are close to
one another, but they are not restricted to
positions that maximize their interactions, so their potential energy is slightly higher than that of a solid. Molecules in the gas phase are far apart, so th
ey interact only weakly, if at
all, and their potential energy is the highest of the three states of matter. Figure 7.13a summarizes the relative energies of the three states of matter.
When heat is added to a substance, the en
ergy increase of the substance can be in the
form of either kinetic or potential energy. In
creases in kinetic energy result in increases in
the thermal energy or temperature of the s
ubstance. However, once certain temperatures
are reached, the heat is used to increase the
potential energy of the substance, not the
kinetic energy. Increases in potential energy r
esult in changes in states; solids melt or
sublime to the higher energy liquid and gas stat
es and liquids evaporate to the gas state.
(Figure 7.13b). However, the state of matter is also a function of pressure; solids also melt and liquids boil as a result of pressure changes.
In this section, we examine how the phases
of a substance vary with both its temperature and its pressure. SOLID-LIQUID TRANSITION The particles in a solid occupy fixed positions that optimize their interactions with one another, and their kinetic energy is in the form of oscillations about these positions. As the temperature is increased, the speed and amplit
ude of the oscillations increase. Eventually,
the oscillations become so agitated and their amplitude so great that the separation between particles becomes sufficiently large
that the forces between them are no longer
strong enough to keep the particles in their fixed positions. At this point, the structure of the solid collapses into a liquid
as the substance melts. The forces that held the particles in
their fixed positions in the solid still exist in
the liquid, but they are weaker because the
particles are farther apart.
Chapter 7 States of Matter and Changes in State
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North
Carolina
State
University