|
Ordinary ice, ice I, is in equilibrium with water vapour and liquid water at +0.0077° C. under a pressure of 4.579 mm. it is always produced when water crystallises spontaneously under pressures less than 2,500 kgm./cm2 Above this pressure ice III or ice II forms; between pressures of 3,500 and 6,300 kgm./cm2 ice II or ice V is produced, according to the temperature; under higher pressures ice VI is formed. Ice II (d = 1.03) cannot exist in equilibrium with liquid water. Ice I, ice II and ice III (d = 1.04) exhibit a triple point at -34.7° C. and 2,170 kgm./cm2; ice II, ice III and ice V (d = 1.09) are in equilibrium at -24.3° C. and 3,510 kgm./cm2. The existence of ice IV is doubtful; ice VI (d = 1.06) is stable only at temperatures above 0°C. Ice II and ice III differ in space lattice only.
The vapour pressures of ice are slightly less than those of supercooled liquid water at the same temperatures. Water cooled below 0° freezes in contact with ice or if violently agitated, and the temperature rises to 0°. Drops of water floating in oil are readily heated much above 100° without vaporising, and are then said to be superheated. ("Superheated steam" is merely steam raised above 100°, the temperature of saturation, by passing through heated tubes.)
Ice crystallises in the hexagonal (six-sided) system.
 | Fig: Snow crystals |
The bubbles in ice are composed of air dissolved in the water, and liberated on freezing. In making clear ice, the freezing is carried out slowly, with agitation, so that the air bubbles have an opportunity to escape.
|
|
