Chapter 7 States of Matter and Changes in State
(^) •
show how to determine if a molecule has a permanent dipole;
(^) •
describe the unique properties of wate
r that result from hydrogen bonding;
(^) •
compare thermal energy to the energy of interaction for a solid, liquid, and gas;
(^) •
describe some properties of liquids and relate
them to the strengths of the intermolecular
interactions;
(^) •
describe the relative energies
of the three states of matter;
(^) •
describe the various changes in state at a molecular level; and
(^) •
demonstrate the use of a phase di
agram and how it is constructed.
7.1
GASES
Molecules in the
gaseous
state adopt the shape and the volume of their closed containers.
If the volume of the container is changed, then
the volume of the gas changes as well. The
focus of this section is the variation of the volume (V) of a gas as its pressure (P), its temperature (T), and its amount (n) are changed.
closed end
h
Patm
PRESSURE AND BOYLE’S LAW Studies of the composition of air and the properties of gases were greatly simplified in the mid 1600’s when an Italian named Evangelista
Torricelli (Galileo’s secretary) invented the
barometer
. Scientists of the time realized that the earth was blanketed with a layer of gas
that had a weight and therefore exerted a pressure, but they did not know how to measure the pressure. Torricelli discovered that when a
glass tube, closed at one end and open at
the other, is filled with mercury and immer
sed with the open end down into a pool of
mercury, only part of the mercury drains fro
m the tube (Figure 7.1). Furthermore, the
height of the mercury column is independent of the length and cross-sectional area of the tube. He reasoned that it was the pressure exerted by the atmosphere that supported the column of mercury in the tube, thus the pressu
re exerted by the weight of mercury in the
tube must equal the pressure exerted by the gases in the atmosphere.
- Increases or
decreases in pressure result in rises or drops in
the height of the column, respectively.
Consequently, the pressure is frequently given by
the height of the column directly, and, in
honor of the inventor of the barometer, a pressure that supports a mercury column to a height of 1 mm is called a
torr
.
Figure 7.1 A barometer The pressure exerted by the weight of the mercury column of height h equals the pressure exerted by the weight of the atmosphere.
*
Pressure is a force per unit area,P = F/A. In a barometer, the forceis the weight of the mercury inthe inverted tube, and A is the cross-sectional area of the tube.The pressure exerted by the atmospheric gases at sea level is 14.7 lb/in
- This means
that a column of the atmosphere that goes fro
m sea level to the top of the atmosphere (~30
km) and has a cross-sectional area 1 squa
re inch weighs 14.7 pounds. Atmospheric
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