116 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
Supercooling and superheating are nonequilibrium
phenomena, and they are different from freezing
point depression and boiling point elevation.
The temperature differences between the freez-
ing and boiling points of solutions and those of pure
water are proportional to the concentration of the so-
lutions. The proportionality constants are molar
freezing point depression(Kf1.86 K mol-1kg)
and the molar boiling point elevation(Kb0.52 K
mol-1kg), respectively. In other words, a solution
containing one mole of nonvolatile molecules per
kilogram of water freezes at 1.86° below 273.15 K,
and boils 0.52° above 373.15 K. For a solution with
concentration C, measured in moles of particles
(molecules and positive and negative ions counted
separately) per kilogram of water, the boiling point
elevation or freezing point depression Tcan be
evaluated.
TKC
where K represents Kf or Kb for freezing point
depression or boiling point elevation, respectively.
This formula applies to both cases. Depending on
the solute, some aqueous solutions may deviate from
Raoult’s law, and the above formulas give only esti-
mates.
In freezing or boiling a solution of a nonvolatile
solute, the solid and vapor contain only H 2 O, leav-
ing the solute in the solution. A solution containing
volatile solutes will have a total vapor pressure due
to all volatile components in the solution. Its boiling
point is no longer that of water alone. Freezing point
depression and boiling point elevation are both relat-
ed to the vapor pressure. In general, a solution of
nonvolatile substance has a lower vapor pressure
than that of pure water. The variations of these prop-
erties have many applications, some in food chem-
istry and biochemistry.
It is interesting to note that some fish and insects
have antifreeze proteins that depress the freezing
point of water to protect them from freezing in the
arctic sea (Marshall et al. 2004).
Osmotic pressureis usually defined as the pres-
sure that must be applied to the side of the solution
to prevent the flow of the pure solvent passing a
semipermeable membrane into the solution. Experi-
mental results show that osmotic pressure is equal to
the product of total concentration of molecules and
ions (C),the gas constant (R 8.3145 J/mol/K),
and the temperature (Tin K):
Osmotic pressure CRT.The expression for osmotic pressure is the same as
that for ideal gas. Chemists calculate the pressure
using a concentration based on mass of the solvent.
Since solutions are never ideal, the formula gives
only estimates. The unit of pressure works out if the
units for Care in mol/m^3 , since 1 J 1 Pa m^3.
Concentration based on mass of solvent differs only
slightly from that based on volume of the solution.
An isotonic solution(isosmotic) is one that has
the same osmotic pressure as another. Solutions
with higher and lower osmotic pressures are called
hypertonic(hyperosmotic), and hypotonic(hypo-
osmotic), respectively. Raw food animal and plant
cells submerged in isotonic solutions with their cell
fluids will not take up or lose water even if the cell
membranes are semipermeable. However, in plant,
soil, and food sciences, water potential gradientis
the driving force or energy directing water move-
ment. Water moves from high-potential sites to low-
potential sites. Water moves from low osmotic pres-
sure solutions to high osmotic pressure solutions.
For consistency and to avoid confusion, negative
osmotic pressure is defined as osmotic potential.
This way, osmotic potential is a direct component of
water potential for gradient consideration. For
example, when red blood cells are placed in dilute
solutions, their osmotic potential is negative. Water
diffuses into the cells, resulting in the swelling or
even bursting of the cells. On the other hand, when
cells are placed in concentrated (hypertonic) saline
solutions, the cells will shrink due to water loss.
Biological membranes are much more permeable
than most man-made phospholipid membranes be-
cause they have specific membrane-bound proteins
acting as water channels. Absorption of water and its
transport throughout the body is more complicated
than osmosis.SOLUTION OFELECTROLYTESSolutions of acids, bases, and salts contain ions.
Charged ions move when driven by an electric po-
tential, and electrolyte solutions conduct electricity.
These ion-containing substances are called elec-
trolytes. As mentioned earlier, the high dielectric
constant of water reduces the attraction of ions with-
in ionic solids and dissolves them. Furthermore, the
polar water molecules surround ions, forming hy-