BioPHYSICAL chemistry

temperatures and pressures. At 1 atm, ice is a stable phase of water below

0°C but above that temperature the liquid phase of water is more stable.

The change in phase indicates that the Gibbs energy of water decreases as

ice changes into liquid water above 0°C. At the transition temperature,

the two phases are in equilibriumas the Gibbs energy is at a minimum

value.

If the temperature of a substance passes through the transition temper-

ature, the substance may not be observed to change phase as the energetics

may be favorable but the kinetics may be too slow to be significant in

practice. Thermodynamically unfavorable phases that are present due to

kinetic limitations are referred to asmetastable phases. Whereas most organ-

isms live only at 1 atm and above 4°C, the isolated components of cells,

such as proteins and lipids, can be found in different phases that are reveal-

ing about their physical natures.

Phase diagrams and transitions

In principle, transitions can occur between two phases. For example, if a

solid block of carbon dioxide is placed in an open container at 1 atm, the

solid will transform into gaseous carbon dioxide without passing through

the liquid phase. The effect of factors such as pressure or temperature

on the phase of a molecule is typically mapped out by the use of phase

diagrams. For example, water will go from ice to liquid and then to vapor

as the temperature increases at a given pressure

(Figure 4.1). On the phase diagram this corres-

ponds to moving horizontally from point A to B

and then C. The melting and boiling temperatures

are dependent upon the pressure. In general,

the vapor phase is favored as the temperature

increases and the pressure decreases. For example,

water boils at a lower temperature at higher

elevations due to lower pressure.

On a phase diagram, the dependence of the

state upon these parameters is shown by identi-

fying what state is present for every temperature

and pressure. A line between two states repres-

ents conditions at which both states can exist

in equilibrium and is called a phase boundary.

Notice that in addition to the vapor/liquid and

liquid/solid phase boundaries there is also a

phase boundary between the solid and vapor

states at low temperatures and pressures. This

latter boundary represents a direct transition

from a solid to a gas without the presence of a

72 PARTI THERMODYNAMICS AND KINETICS

Water

Ice

Figure 4.1In
response to heating,
the temperature of
ice increases, leading
to a transition to
the liquid phase,
followed by a
transition to the
vapor phase.