Molecules can exist in three different states, as a solid, liquid, or gas. Changes
in these states, or phases, are commonplace and so a description of the
factors influencing the transitions plays a critical role in helping us to
understand the properties of molecules. In this chapter, we examine the
thermodynamic conditions that determine which phase is present and when
a phase change occurs. For example, water can exist as ice, liquid, or a
gas, and temperature is a key determinant of which phase is present, with
water forming ice in the freezer and water changing into vapor when
heated in the oven. Once the thermodynamic factors that determine the
phase of pure, simple molecules have been discussed, we will turn our
attention to the more complex questions such as what factors cause lipids
to form cell membranes and proteins to crystallize.
Substances may exist in different phases
A substance is described as having a form of matter termed a phase
when the substance is uniform in chemical composition and physical state.
For example, a substance may exist in solid, liquid, or gases phases.
Phases may co-exist, as a cup of ice water consists of two distinct phases,
each of which is uniform. A substance may exist in many different solid
phases depending upon the conditions. For example, as the pressure is
increased on water below 0°C, different structural forms of ice are stable
as the hydrogen bonds between water molecules are modified by the
stress arising from the pressure. Most substances have only a single
liquid state, although there are exceptions, and all substances exist in a
single gaseous phase.
Substances may make a phase transition, which is aspontaneous con 9 ersion
from one phase into another phase. Phase transitions occur at characteristic