Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 113

water density between the freezing point and the
boiling point is shown in Figure 5.8. The densities
are 0.9998426, 0.9999750, and 0.9998509 Mg m^3
at 0, 4, and 8°C, respectively. The decrease in densi-
ty is not linear, and at 100°C, the density is 0.95840
Mg m^3 , a decrease of 4% from its maximum.

Vapor Pressure of Liquid H 2 O

Equilibrium vapor pressure of water, Figure 5.9,
increases with temperature, similar to that of ice. At
the triple point, the vapor pressures of ice Ih and
water are the same, 0.611 kPa, and the boiling point
(373.15 K, 100°C) is the temperature at which the
vapor pressure is 101.325 kPa (1 atm). At slightly
below 394 K (121°C), the vapor pressure is 202.65
kPa (2.00 atm). At 473 and 574 K, the vapor pres-
sures are 1553.6 and 8583.8 kPa, respectively. The
vapor pressure rises rapidly as temperature increas-
es. The lowest pressure to liquefy vapor just below
the critical temperature, 373.98°C, is 22,055 kPa
(217.67 atm), and this is known as the critical pres-
sure. Above 373.98°C, water cannot be liquefied,
and the fluid is called supercritical water.
The partial pressure of H 2 O in the air at any tem-
perature is the absolute humidity. When the partial
pressure of water vapor in the air is the equilibrium
vapor pressure of water at the same temperature, the
relative humidityis 100%, and the air is saturated

with water vapor. The partial vapor pressure in the

air divided by the equilibrium vapor pressure of

water at the temperature of the air is the relative

humidity, expressed as a percentage. The tempera-

ture at which the vapor pressure in the air becomes

saturated is the dew point, at which dew begins to

form. Of course when the dew point is below 273 K

or 0°C, ice crystals (frost) begin to form. Thus, the

relative humidity can be measured by finding the

dew point and then dividing the equilibrium vapor

pressure at the dew point by the equilibrium vapor

pressure of water at the temperature of the air. The

transformations between solid, liquid, and gaseous

water play important roles in hydrology and in trans-

forming the earth’s surface. Solar energy causes

phase transitions of water that make the weather.

TRANSFORMATION OFSOLID, LIQUID, AND

VAPOR

Food processing and biochemistry involve transfor-

mations among solid, liquid, and vapor of water.

Therefore, it is important to understand ice-water,

ice-vapor, and water-vapor transformations and their

equilibria. These transformations affect our daily

lives as well. A map or diagram is helpful in order to

comprehend these natural phenomena. Such a map,

representing or explaining these transformations, is

called a phase diagram(see Fig. 5.10). A sketch

Figure 5.8.Density of water Mg m^3 as a function of
temperature (°C).
Figure 5.9.Equilibrium vapor pressure of water as a
function of temperature.