SUMMARY OF GAS LAWS
Chapter 8 Solids, Liquids, and Gases
Opening Essay
We normally experience carbon dioxide (CO2) as a gas, but if it were cooled down to
about −78°C, it would become a solid. The everyday term for solid carbon dioxide is
dry ice.
Why “dry” ice? Solid carbon dioxide is called dry ice because it converts from a solid to
a gas directly, without going through the liquid phase, in a process called sublimation.
Thus, there is no messy liquid phase to worry about. Although it is a novelty, dry ice
has some potential dangers. Because it is so cold, it can freeze living tissues very
quickly, so people handling dry ice should wear special protective gloves. The cold
carbon dioxide gas is also heavier than air (because it is cold and more dense), so
people in the presence of dry ice should be in a well-ventilated area.
Dry ice has several common uses. Because it is so cold, it is used as a refrigerant to
keep other things cold or frozen (e.g., meats or ice cream). In the medical field, dry ice
is used to preserve medical specimens, blood products, and drugs. It also has
dermatological applications (e.g., freezing off warts). Organs for transplant are kept
cool with dry ice until the recipient of the new organ is ready for surgery. In this
respect, carbon dioxide is much like water—more than one phase of the same
substance has significant uses in the real world.
Most of us are familiar with the three phases of matter: solid, liquid, and gas. Indeed,
we addressed the energy changes involved in phase changes in Chapter 7 "Energy and
Chemical Processes". The picture on this page shows the substance we are probably
most familiar with as having those three phases: water. In everyday life, we commonly
come in contact with water as a solid (ice), as a liquid, and as a gas (steam). All we
,have to do is change the conditions of the substance—typically temperature—and we
can change the phase from solid to liquid to gas and back again.
Under the proper conditions of temperature and pressure, many substances—not only
water—can experience the three different phases (Figure 8.1 "Water"). An
understanding of the phases of matter is important for our understanding of all matter.
In this chapter, we will explore the three phases of matter.
Figure 8.1 Water
Water is probably the most familiar substance that commonly exhibits in all three
phases. However, many substances will exhibit the solid, liquid, and gas phases under
certain conditions. For example, in clouds, liquid water exists as tiny droplets
condensed from water vapor in the air.
© Thinkstock
,8.1 Intermolecular Interactions
Learning Objectives
1. Define phase.
2. Identify the types of interactions between molecules.
A phase is a certain form of matter that includes a specific set of physical properties.
That is, the atoms, the molecules, or the ions that make up the phase do so in a
consistent manner throughout the phase. As mentioned in Chapter 1 "Chemistry,
Matter, and Measurement", science recognizes three stable phases: the solid phase, in
which individual particles can be thought of as in contact and held in place; the liquid
phase, in which individual particles are in contact but moving with respect to each
other; and the gas phase, in which individual particles are separated from each other
by relatively large distances. Not all substances will readily exhibit all phases. For
example, carbon dioxide does not exhibit a liquid phase unless the pressure is greater
than about six times normal atmospheric pressure. Other substances, especially
complex organic molecules, may decompose at higher temperatures, rather than
becoming a liquid or a gas.
Note
For many substances, there are different arrangements the particles can take in the
solid phase, depending on temperature and pressure.
, Which phase a substance adopts depends on the pressure and the temperature it
experiences. Of these two conditions, temperature variations are more obviously
related to the phase of a substance. When it is very cold, H 2O exists in the solid form as
ice. When it is warmer, the liquid phase of H2O is present. At even higher
temperatures, H2O boils and becomes steam.
Pressure changes can also affect the presence of a particular phase (as we indicated
for carbon dioxide), but its effects are less obvious most of the time. We will mostly
focus on the temperature effects on phases, mentioning pressure effects only when
they are important. Most chemical substances follow the same pattern of phases when
going from a low temperature to a high temperature: the solid phase, then the liquid
phase, and then the gas phase. However, the temperatures at which these phases are
present differ for all substances and can be rather extreme. Table 8.1 "Temperature
Ranges for the Three Phases of Various Substances" shows the temperature ranges for
solid, liquid, and gas phases for three substances. As you can see, there is extreme
variability in the temperature ranges.
Table 8.1 Temperature Ranges for the Three Phases of Various Substances
Substance Solid Phase Below Liquid Phase Above Gas Phase Above
hydrogen (H2) −259°C −259°C −253°C
water (H2O) 0°C 0°C 100°C
sodium chloride (NaCl) 801°C 801°C 1413°C
The melting point of a substance is the temperature that separates a solid and a liquid. The boiling
point of a substance is the temperature that separates a liquid and a gas.
What accounts for this variability? Why do some substances become liquids at very low
temperatures, while others require very high temperatures before they become
liquids? It all depends on the strength of the intermolecular interactions between the
particles of substances. (Although ionic compounds are not composed of discrete
molecules, we will still use the term intermolecular to include interactions between the
Chapter 8 Solids, Liquids, and Gases
Opening Essay
We normally experience carbon dioxide (CO2) as a gas, but if it were cooled down to
about −78°C, it would become a solid. The everyday term for solid carbon dioxide is
dry ice.
Why “dry” ice? Solid carbon dioxide is called dry ice because it converts from a solid to
a gas directly, without going through the liquid phase, in a process called sublimation.
Thus, there is no messy liquid phase to worry about. Although it is a novelty, dry ice
has some potential dangers. Because it is so cold, it can freeze living tissues very
quickly, so people handling dry ice should wear special protective gloves. The cold
carbon dioxide gas is also heavier than air (because it is cold and more dense), so
people in the presence of dry ice should be in a well-ventilated area.
Dry ice has several common uses. Because it is so cold, it is used as a refrigerant to
keep other things cold or frozen (e.g., meats or ice cream). In the medical field, dry ice
is used to preserve medical specimens, blood products, and drugs. It also has
dermatological applications (e.g., freezing off warts). Organs for transplant are kept
cool with dry ice until the recipient of the new organ is ready for surgery. In this
respect, carbon dioxide is much like water—more than one phase of the same
substance has significant uses in the real world.
Most of us are familiar with the three phases of matter: solid, liquid, and gas. Indeed,
we addressed the energy changes involved in phase changes in Chapter 7 "Energy and
Chemical Processes". The picture on this page shows the substance we are probably
most familiar with as having those three phases: water. In everyday life, we commonly
come in contact with water as a solid (ice), as a liquid, and as a gas (steam). All we
,have to do is change the conditions of the substance—typically temperature—and we
can change the phase from solid to liquid to gas and back again.
Under the proper conditions of temperature and pressure, many substances—not only
water—can experience the three different phases (Figure 8.1 "Water"). An
understanding of the phases of matter is important for our understanding of all matter.
In this chapter, we will explore the three phases of matter.
Figure 8.1 Water
Water is probably the most familiar substance that commonly exhibits in all three
phases. However, many substances will exhibit the solid, liquid, and gas phases under
certain conditions. For example, in clouds, liquid water exists as tiny droplets
condensed from water vapor in the air.
© Thinkstock
,8.1 Intermolecular Interactions
Learning Objectives
1. Define phase.
2. Identify the types of interactions between molecules.
A phase is a certain form of matter that includes a specific set of physical properties.
That is, the atoms, the molecules, or the ions that make up the phase do so in a
consistent manner throughout the phase. As mentioned in Chapter 1 "Chemistry,
Matter, and Measurement", science recognizes three stable phases: the solid phase, in
which individual particles can be thought of as in contact and held in place; the liquid
phase, in which individual particles are in contact but moving with respect to each
other; and the gas phase, in which individual particles are separated from each other
by relatively large distances. Not all substances will readily exhibit all phases. For
example, carbon dioxide does not exhibit a liquid phase unless the pressure is greater
than about six times normal atmospheric pressure. Other substances, especially
complex organic molecules, may decompose at higher temperatures, rather than
becoming a liquid or a gas.
Note
For many substances, there are different arrangements the particles can take in the
solid phase, depending on temperature and pressure.
, Which phase a substance adopts depends on the pressure and the temperature it
experiences. Of these two conditions, temperature variations are more obviously
related to the phase of a substance. When it is very cold, H 2O exists in the solid form as
ice. When it is warmer, the liquid phase of H2O is present. At even higher
temperatures, H2O boils and becomes steam.
Pressure changes can also affect the presence of a particular phase (as we indicated
for carbon dioxide), but its effects are less obvious most of the time. We will mostly
focus on the temperature effects on phases, mentioning pressure effects only when
they are important. Most chemical substances follow the same pattern of phases when
going from a low temperature to a high temperature: the solid phase, then the liquid
phase, and then the gas phase. However, the temperatures at which these phases are
present differ for all substances and can be rather extreme. Table 8.1 "Temperature
Ranges for the Three Phases of Various Substances" shows the temperature ranges for
solid, liquid, and gas phases for three substances. As you can see, there is extreme
variability in the temperature ranges.
Table 8.1 Temperature Ranges for the Three Phases of Various Substances
Substance Solid Phase Below Liquid Phase Above Gas Phase Above
hydrogen (H2) −259°C −259°C −253°C
water (H2O) 0°C 0°C 100°C
sodium chloride (NaCl) 801°C 801°C 1413°C
The melting point of a substance is the temperature that separates a solid and a liquid. The boiling
point of a substance is the temperature that separates a liquid and a gas.
What accounts for this variability? Why do some substances become liquids at very low
temperatures, while others require very high temperatures before they become
liquids? It all depends on the strength of the intermolecular interactions between the
particles of substances. (Although ionic compounds are not composed of discrete
molecules, we will still use the term intermolecular to include interactions between the
