Ice is frozen water. Low temperatures cause ice to form on lakes and rivers and on wet streets and sidewalks. Snow, sleet, frost, and hail are ice, and so are glaciers. Even in summer, ice may be present in high clouds. High clouds are composed of ice crystals.
Ice is plentiful throughout the universe. Ice is the main component of comets, which are much like dirty, loosely packed snowballs. Some of Jupiter’s moons, including Europa and Ganymede, may have layers of surface ice.
Properties of ice.
Pure water freezes at 32 °F (0 °C). Water that contains other substances, such as alcohol, salt, or sugar, freezes at a lower temperature. For this reason, road crews put salt or other chemicals on icy streets to melt the ice and make the roads less slippery.
Ice is slippery because, even at temperatures slightly below 32 °F, it has a thin, liquidlike surface. Unlike the molecules within the crystal, the surface molecules are arranged in a disorderly way, and they vibrate rapidly. The liquidlike layer thus acts as a lubricant.
Ice attaches itself strongly to objects on which it forms. This property can be an annoyance when ice sticks to an automobile windshield. It can be a danger when ice sticks to airplane wings.
One property of ice is rare in nature: A volume of ice weighs less than the same volume of water. That is, ice is less dense than water. By contrast, the solid form of most substances is denser than the liquid form. Most liquids contract (decrease in volume) as they cool and freeze. When pure water cools below 39 °F (4 °C), however, it expands until it freezes at 32 °F (0 °C). Because of ice’s low density, it floats on water instead of sinking. Without this property, ice would form on the bottom, rather than on the top, of lakes and rivers.
Water’s changes in volume can have harmful results. The expansion of freezing water in pipes may cause the pipes to burst. The expansion and contraction of freezing and thawing water within the pavement of a road can cause the pavement to crumble.
The structure of ice
is responsible for the expansion of water at low temperatures. Pure ice has a regular crystal structure of frozen water molecules. Each molecule consists of one oxygen (O) and two hydrogen (H) atoms, or H2O. Each molecule is attached to four neighboring molecules by connections known as hydrogen bonds. The bonds extend from the oxygen atom of one molecule to the hydrogen atom of another. Each oxygen atom can be part of two hydrogen bonds, and each hydrogen atom can participate in one hydrogen bond. Thus, each molecule can form four bonds. The bonds are long, and they point away from one another.
Molecules of pure water behave normally until they are cooled below 39 °F. When any liquid is warm, the energy of its molecules causes the molecules to collide. The recoil from the collisions tends to keep the molecules a certain distance apart. As the liquid cools, the molecules become less active. Collisions occur less often and are less energetic. As a result, the molecules draw closer together, and the liquid contracts.
Most liquids continue to contract when cooled below 39 °F. As water is cooled below that temperature, however, hydrogen bonds begin to form, pushing molecules apart. The water expands. When water freezes to form ice, the network of hydrogen bonds is completed. The water suddenly expands further and hardens.
Ice sticks strongly to other objects because the molecules on the surface of the freezing water can form only three hydrogen bonds with other water molecules. The surface molecules form the fourth bond with molecules of the other object.
The length and direction of ice’s hydrogen bonds give ice crystals an open structure. Because of this structure, ice deforms when squeezed. When scientists subject ice to tremendous pressures in the laboratory, the ice forms crystal structures that are more compact than ordinary.
Ice “cages.”
Much ice contains small molecules of other substances trapped within its open crystal structure. This kind of ice is known as a clathrate hydrate << KLATH rayt HY drayt >> or a gas hydrate. Large deposits of ice containing carbon dioxide, natural gas, and other substances occur on the ocean floor. Many scientists believe that natural gas hydrates may provide a future source of energy.
Much of the ice that covers polar regions consists of clathrate hydrates. Climate researchers have drilled cores from the ice and analyzed the gas molecules trapped at different depths. In general, deeper material was deposited earlier. The cores thus provide a record of climate changes, volcanic eruptions, and other events of the past 100,000 years that produced the gases.
Although large deposits of clathrate hydrates exist, ice more commonly excludes impurities as it crystallizes, even in highly contaminated water. Water molecules tend to bond to one another, rather than to impurities. Because of this tendency, ice formed from seawater contains almost no salt. Thus, freezing is a valuable process for the purification of water.
