Chemistry Reference
In-Depth Information
Table 14.1
Properties of the Particles of Solids, Liquids, and Gases
Solids
Liquids
Gases
Strength of attractive
Strong
Moderately
Negligible
forces
strong
Components
Molecules or bonded
Molecules
Molecules
atoms or ions
Distance between particles
Touching
Touching
Far apart
Permanence of position
Permanent
Variable
Variable
Gases are characterized by having little or no forces of attraction between their
molecules (Section 12.10). In contrast, the forces between the particles of which
liquids and solids are composed are significant. These forces range from ionic
or covalent bonds, which are strong, to much weaker intermolecular forces.
Intermolecular forces were discussed in Section 13.6. Substances that are liq-
uid or gaseous at room temperature are composed of molecules. In contrast,
solids may be composed of (1) macromolecules (atoms covalently bonded
together in large networks), (2) ions bonded together, (3) molecules, or
(4) atoms bonded together by loosely held valence electrons. The particles that
make up solids are held more or less stationary in their positions; the molecules
that make up liquids and gases are much freer to move about. In gases, the dis-
tance between molecules is very large compared with the size of the molecules
themselves; in liquids and solids, the particles are essentially touching one
another. These characteristics are summarized in Table 14.1.
Section 14.1 examines the liquid and solid states. Section 14.2 discusses
changes of phase and concepts related to systems with two phases in contact,
such as vapor pressure. In Section 14.3, the measurement of quantities of heat
added to or removed from a system is introduced, and the energies involved in
phase changes are calculated. Section 14.4 deals with the enthalpies involved
in chemical reactions.
14.1
Nature of the Solid and Liquid States
The Solid State
Solids are classified as
crystalline solids
or
amorphous solids.
Crystalline
solids, such as an ice cube or a sodium chloride crystal, have a definite melt-
ing point. Amorphous solids, such as a chocolate bar or glass, get softer and
softer as the temperature is raised. The structures of crystalline solids feature
regularly repeating arrangements of the constituent particles. The structure of
amorphous solids is not regular, but something like that of liquids; sometimes,
amorphous solids are called “supercooled liquids.”
Crystalline solids may be classified as (1)
ionic solids,
in which the repeat-
ing units are ions; (2)
network solids
(or macromolecular solids), in which cova-
lently bonded atoms are the repeating units; (3)
molecular solids,
in which
individual molecules are the repeating units; and (4)
metallic solids,
in which
individual metal atoms are held together by their loosely held valence electrons.
