Ice

Ice is water (H₂O) in solid form. It forms when the temperature of water drops below the freezing point, which under standard atmospheric pressure at sea level is 0 degrees Celsius (32 degrees Fahrenheit). Ice is among the most abundant solids visible on Earth's surface, particularly in polar regions and high mountains.

The most notable property of ice is that it is less dense than liquid water, causing it to float. This distinguishes it from many other substances, whose solid form is denser than their liquid form.

Molecular structure

Water molecules are held together by bonds called hydrogen bonds. When water freezes, these bonds arrange the molecules in a hexagonal crystalline structure with significant empty space. This empty space within ice causes its volume to be greater than that of an equal mass of water—which is why ice floats and water expands when it freezes.

Ice on Earth

Most of Earth's ice is held in ice sheets in Antarctica and Greenland, as well as in mountain glaciers and sea ice in the Arctic Ocean. Sea ice forms from fresh water and differs from terrestrial ice that comes from accumulated snow. When sea ice melts, sea level does not rise, since it was already floating; when terrestrial ice melts, it adds new water to the ocean.

Role in climate

Ice and snow have a white color that reflects much of the sun's radiation (high albedo). As ice diminishes, the darker land or ocean beneath absorbs more heat, which further increases warming. This self-reinforcing process is one of the phenomena that climate scientists monitor.

Uses

Humans use ice for cooling food and beverages, treating inflammation and injuries, and sports such as ice skating and ice hockey. Before mechanical refrigeration was available, natural ice was harvested in summer and stored in dedicated structures.