The Foundations of Chemistry

COMPARISON OF SOLIDS, LIQUIDS, AND GASES

Matter exists in three physical states: solids, liquids, and gases. In the solid state H 2 O is
known as ice, in the liquid state it is called water, and in the gaseous state it is known as
steam or water vapor. Most, but not all, substances can exist in all three states. Most solids
change to liquids and most liquids change to gases as they are heated. Liquids and gases
are known as fluidsbecause they flow freely. Solids and liquids are referred to as
condensed statesbecause they have much higher densities than gases. Table 12-1 displays
the densities of a few common substances in different physical states.
As the data in Table 12-1 indicate, solids and liquids are many times denser than gases.
The molecules must be very far apart in gases and much closer together in liquids and
solids. For example, the volume of one mole of liquid water is about 18 milliliters, whereas
one mole of steam occupies about 30,600 milliliters at 100°C and atmospheric pressure.
Gases are easily compressed, and they completely fill any container in which they are
present. This tells us that the molecules in a gas are far apart relative to their sizes and
that interactions among them are weak. The possibilities for interaction among gaseous
molecules would be minimal (because they are so far apart) were it not for their rapid
motion.
All substances that are gases at room temperature may be liquefied by cooling and
compressing them. Volatile liquids are easily converted to gases at room temperature or
slightly above. The term vaporrefers to a gas that is formed by evaporation of a liquid
or sublimation of a solid. We often use this term when some of the liquid or solid remains
in contact with the gas.

COMPOSITION OF THE ATMOSPHERE AND

SOME COMMON PROPERTIES OF GASES

Many important chemical substances are gases at ambient conditions. The earth’s atmo-
sphere is a mixture of gases and particles of liquids and solids (Table 12-2). The major
gaseous components are N 2 (bp
195.79°C) and O 2 (bp
182.98°C), with smaller concen-
trations of other gases. All gases are miscible; that is, they mix completely unlessthey react
with one another.
Several scientists, notably Torricelli (1643), Boyle (1660), Charles (1787), and Graham
(1831), laid an experimental foundation on which our present understanding of gases is
based. For example, their investigations showed that

12-2

12-1

Some compounds decompose before

melting or boiling.

Ice is less dense than liquid water. This

behavior is quite unusual; most

substances are denser in the solid state

than in the liquid state.

12-2 Composition of the Atmosphere and Some Common Properties of Gases 435

TABLE 12-1 Densities and Molar Volumes of Three Substances at Atmospheric Pressure*

Substance Solid Liquid (20°C) Gas (100°C)

Density Molar volume Density Molar volume Density Molar volume

(g/mL) (mL/mol) (g/mL) (mL/mol) (g/mL) (mL/mol)

water (H 2 O) 0.917 (0°C) 19.6 0.998 18.0 0.000588 30,600
benzene (C 6 H 6 ) 0.899 (0°C) 86.9 0.876 89.2 0.00255 30,600

carbon tetrachloride (CCl 4 ) 1.70 (
25°C) 90.5 1.59 96.8 0.00503 30,600

*The molar volume of a substance is the volume occupied by one mole of that substance.

Volatileliquids evaporate readily. They

have low boiling points.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 1.3, States of Matter.

TABLE 12-2 Composition

of Dry Air

Gas % by Volume

N 2 78.09

O 2 20.94

Ar 0.93

CO 2 0.03*

He, Ne, Kr, Xe 0.002

CH 4 0.00015*

H 2 0.00005

All others combined† 0.00004

*Variable.

†Atmospheric moisture varies.