Ice, frozen water. (Dry ice is not frozen water, but carbon dioxide in solid form) Under ordinary conditions, water changes to ice at 32 F. (0 C.). Ice is cold, hard, brittle, and slippery.
Ice occurs in many forms. The largest forms are glaciers (including polar ice caps), occurring on land; and packice, or the frozen waters of seas, lakes, and rivers. Icebergs are floating masses of ice that have broken off from glaciers. Pack ice breaks up into ice floes.
Other forms of ice include ice crystals that form within clouds, even in tropical regions; the snow and hail that fall from clouds; and sleet, which occurs when raindrops falling from the upper atmosphere pass into colder air and freeze. During cold weather, a coating of ice called frost may cover the ground and trace beautiful patterns on windowpanes. Sometimes a thin coating, or glaze, of ice appears on roads and trees.
The land surface of Canada, the northern United States, and certain other regions of the Northern Hemisphere was given its present appearance by the action of enormous ice sheets during the Glacial Period. Ice has a continuing geological effect in the weathering of rocks and in erosion.
Before the development of mechanical refrigeration, ice was used to preserve perishable foods, both in the home and during long-distance transportation. Today, its main domestic use is in cold drinks and in portable ice chests for picnics. Ice packs are used to reduce fever and pain. Ice sports include skating, hockey, and iceboating.
The hazard of icy walks and streets is reduced by sanding and salting. Aircraft, which may be disabled if they become heavily coated with ice, are equipped with deicing devices. Ice patrols watch for icebergs, and sturdy icebreakers clear a path through river, lake, and ocean ice for other vessels.
Because water expands when it freezes, ice may burst plumbing, or crack the radiator or engine block of an automobile. To prevent such damage, pipes are laid in the ground below the frost line, and antifreeze is added to an automobile's cooling system.
When water at its freezing point, 32 F. (0 C.), is cooled it turns into ice. As water turns into ice it expands and the volume of the water increases by 112. As ice forms the molecules of water arrange themselves into a framework; the expansion occurs because in this framework the average distance between the molecules is greater than it is between the molecules in liquid water. Water is one of the few substances that expand when they freeze. (Among the others are bismuth and iron.)
The density (mass per unit volume) of ice is about 92 per cent that of water in liquid form; ice, therefore, floats in water, though it remains largely submerged. The fact that ice floats has important consequences, as discussed in water, subtitle Physical Properties of Water.
Ice melts when the temperature rises above 32 F. (0 C.), under normal atmospheric pressure. Additional pressure of any kind lowers the melting point of ice. For example, ice melts slightly under a person's weight and refreezes as soon as the pressure is removed. This effect is called regelation. Ice feels slippery because it has a thin coating of water, the result of surface melting due to pressure, friction, or the heat of the sun or air.
The specific heat of ice is one-half of a calorie. This means that one-half calorie of heat is required to raise the temperature of one gram of ice one degree Celsius. The latent heat of ice is 80 calories. Thus, a gram of ice absorbs 80 calories in melting and gives off 80 calories in freezing.
Saltwater has a lower freezing point than freshwater. Seawater, for example, freezes at about 27 F. (–3 C.). In winter, salt is sometimes put into the water used in mixing concrete, to prevent the concrete from freezing. Salt is sometimes sprinkled on streets and highways after a snowfall, to hasten melting.
Block, or can, ice is made in slightly tapered rectangular cans submerged in a large tank of moving brine at a temperature between 12 and 18 F. (–11 and –8 C.). Each can contains 300 to 400 pounds (135 to 180 kg) of pure water. After the ice has formed, the cans are removed and dipped in warm water to free the ice.
Cracked, or plate, ice is made by crushing thin sheets of ice. The sheets are frozen on refrigerated plates arranged vertically in an electrically operated machine.
Flakes or fine chips of ice are made in a cylindrical machine containing cold brine in a rotating drum. A thin sheet of ice forms on the outside of the drum and is continuously scraped away. Small versions of this machine are used in hotels, clubs, and hospitals. Ice cubes are made in similar machines and in home refrigerators.
The ice of indoor skating rinks is formed by spraying water on a floor containing pipes filled with cold brine. When a new ice surface is needed, the brine is drained, and steam sent through the pipes melts the ice.