Ice

I

Introduction

Ice, water in the solid state. Water ice is colorless and transparent, and crystallizes in the hexagonal (six-sided) system, as seen in snowflakes (see Crystal). Its melting point is 0°C (32°F). Pure water also freezes at 0°C, but ice will form at 0°C only if the water is disturbed or contaminated with dust or other objects. Ice plays a major role in weather and climate on Earth. Frozen forms of other substances, such as carbon dioxide, ammonia, and methane, are also known as ices.

II

Properties

One important property of water ice is that it expands upon freezing. At 0°C it has a specific gravity of 0.9168 as compared to the specific gravity of water—0.9998—at the same temperature. As a result, ice floats in water.

Because water expands when it freezes, an increase of pressure tends to change ice into water and therefore lowers the melting point of ice. This effect is not very marked for ordinary increases of pressure. For instance, at a pressure 100 times the normal atmospheric pressure, the melting point of ice is only about 1°C (about 1.8°F) less than at normal pressure.

At higher pressures, however, several allotropic modifications, or allotropes (different forms of the same element that exist in the same physical state) of ice are formed. These are designated Ice II, Ice III, Ice V, Ice VI, and Ice VII. Ordinary ice is Ice I. These allotropes are denser than water and their melting points rise with increased pressure. At about 6,000 atmospheres (about 6,200 kg per sq cm, or about 88,200 lb per sq in) the melting point is again 0°C and at a pressure of 20,000 atmospheres (about 20,700 kg per sq cm, or about 294,000 lb per sq in) the melting point rises above 80°C (176°F).

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III

Frozen Water on Earth

A

Bodies of Fresh Water

The special properties of freezing water explain the way in which open bodies of fresh water freeze. When the temperature of the surface of an open body of water such as a lake, a pond, or a river is reduced toward the freezing point, the surface water becomes denser as it cools, and therefore sinks. It is replaced at the surface by warmer water from beneath. Eventually the entire body of water reaches a uniform temperature of 4.0°C (39.2°F), the point at which water has its maximum density. If the water is cooled further, its density decreases and finally ice is formed on the surface. Bodies of water freeze from the top down rather than from the bottom up because of these density differences.

In rivers, however, ice is sometimes formed beneath the surface. On cold winter nights the surface of a swiftly flowing stream may become cooled well below 0°C because of its contact with the air. Such “undercooled” water, mixing with the warmer layers beneath, produces a spongy mass of ice crystals known as frazil, which floats downstream. Sometimes masses of frazil lodging under surface ice in quieter water may dam a stream and cause floods.

Another form of below-surface ice is anchor ice, which is formed around rocks on streambeds. During cold nights enough heat may be radiated from the rocks so that they become cool enough to freeze the water flowing around them. When the rocks are warmed by the sun in the daytime, masses of the anchor ice may detach and rise to the surface of the stream.

B

Sea Ice

Water at the surface of the ocean freezes in winter in the Arctic and Antarctic regions, forming sea ice. Seawater freezes differently from the way fresh water freezes, in a much slower process and at a lower temperature (-1.8°C, or 28.8°F). Salt makes the density of seawater increase as it approaches its freezing point, and very cold seawater sinks. Sea ice does not begin to form until the top layer of the ocean has cooled sufficiently to a depth of about 100 to 150 m (300 to 450 ft).

As seawater freezes into sea ice, the salt is expelled to form tiny droplets of concentrated liquid brine surrounded by crystals of frozen fresh water. Over time, the dense brine drains downward and sinks into the ocean from the underside of the sea ice. Snow that accumulates and melts on the sea ice may also help flush the salt downward. As a result of these processes, sea ice that has lasted for a number of years is nearly pure fresh water and is suitable for drinking—unlike newly formed sea ice, which is as salty as seawater when melted.

Sea ice plays an important role in the natural environment of the Arctic and Antarctic. Many polar animals such as polar bears, walrus, seals, and penguins depend on sea ice for places to hunt, rest, take shelter, or give birth. In the Arctic, female walrus nurse their young and leave them on blocks of floating sea ice. In the Antarctic, the emperor penguin breeds on stable platforms of sea ice during the winter.

Sea ice at the poles influences climate in many ways. Some amount of sea ice survives through the summer, reflecting sunlight that would otherwise warm dark, open water (see Albedo). As salt water drains from sea ice, the denser salty water underneath the ice sinks, creating cold currents in the ocean. Sea ice also reduces evaporation off the surface of the ocean. Without sea ice, more water vapor can enter the atmosphere, resulting in more frequent and stronger storms. Many scientists think global warming is responsible for the reduction and thinning of sea ice recorded in recent years.

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