Matters of state and motion - Physical Processes in Earth and Environmental Sciences - page 18

2 Matters of state and
motion

NERGY is required
plate motion, water
weather, and

SOLA

2.1 Matters of state

Earth contains each of the states of matter - solid, liquid
and gas (Fig. 2.1); we have noted already that Earth's
surface is unique among the planets of the Solar System
in its abundance of solid, liquid, and gaseous water. The
mass and energy transfers that accompany changes of
state of water dominate physical conditions in the tropos-
phere, hydrosphere, and cryosphere and have great
importance in modulating more “rocky” processes such
as flow and partial melting in the upper lithosphere. Yet
matters of state are more subtle and interesting than just
a simple threefold division; for example, consider the
initially rather strange idea of granular fluids noted
below.

populated by relatively few individual molecules in
constant random high-speed motion. The spaciousness of
gases explains their low density and high compressibility.
Gas pressure is a measure of the intensity of the random
collisions of gas molecules with some rigid wall or con-
tainer. Thermal properties include low conductivity and
low specific heat capacity. These result from the ineffi-
ciency of kinetic energy exchange due to rare intermolecular
collisions and the lack of transmittable molecular oscilla-
tions and rotations to cause conduction of heat energy as
temperature is increased. Gases have low potential energies
due to their lack of interaction with neighboring molecules.
They are thus good insulators and poor heat carriers.

2.1.1 Gases 2.1.2 Liquids

Liquids are typically of the order of 10 3 times denser than
gases, with low compressibility and no rigidity. Liquid
molecules within a given volume are relatively close

The physical properties of gases are low density, absence of
rigidity, and very high compressibility. These reflect the
“openness” of space within a gas volume (Figs 2.2 and 2.3),

Fig. 2.1 States of water. Joule's famous “doodle,” done in an 1847 notebook, of the three states of matter for H 2 O. Sketched as he realized that
gas molecules must be the most widely spaced and have independent motions. Around this time, Avogadro's number was becoming reliably
known. Its huge magnitude (there are 6 10 23 molecules in a single mole of every substance) and the tiny size of atoms gives the appearance

of continuous matter at the scale of human eyesight. This continuum approach is still the most useful for analysis of fluid and solid properties
in the bulk.