ice
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What is ice?
Ice is water in its solid state, formed when water freezes at or below 0 °C (32 °F). It can appear as frost, snowflakes, river ice, sea ice, or hail.
The water molecule that makes up ice consists of two hydrogen atoms and one oxygen atom (H2O). These atoms form a specific structure, with the oxygen atom between the two hydrogen atoms at an angle of about 105°. This arrangement leads to hydrogen bonds, which give ice its rigid, crystalline structure.
Ice is less dense than liquid water, which is why it floats. It can exist in various forms, such as glaciers, icebergs, and sea ice. Sea ice can be first-year ice (not more than one winter old) or multiyear ice (surviving one or more summers).
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How is ice formed from water?
Ice forms when water reaches its freezing point at 0 °C (32 °F). Water molecules, each comprising two hydrogen atoms and one oxygen atom (H2O), bond covalently. In liquid form, these molecules form chains linked by hydrogen bonds, constantly breaking and reforming due to thermal agitation.
As water cools and freezes, these molecules arrange themselves into an ordered, crystalline structure, forming open hexagonal rings. This lattice structure increases the space between the molecules, which reduces density, causing ice to be less dense than liquid water. This is why ice floats. In cold clouds, ice crystals can also form by deposition of water vapor onto existing ice crystals or by the freezing of supercooled water droplets.
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At what temperature does water freeze into ice?
Water freezes into ice at 0 °C (32 °F) or lower. The Celsius scale is based on 0° for the freezing point of water and 100° for the boiling point. The Fahrenheit scale defines the freezing point of water as 32° and the boiling point as 212°.
When water cools from 0 to 4 °C, it becomes denser, and open-structured molecular arrangements break down, reducing their volume to that of more closely packed polymeric structures of the liquid state.
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What happens to water molecules when they turn into ice?
When water molecules transition into ice, they arrange themselves into an ordered, crystalline structure where each oxygen atom is linked to four hydrogen atoms. Two hydrogen atoms form covalent bonds with the oxygen atom, while the other two form hydrogen bonds.
This arrangement forms an open, hexagonal lattice, which increases the space between the molecules, thus reducing density compared to liquid water. This open structure is the reason why ice is less dense than liquid water, allowing it to float.
The ice crystal commonly forms as sheets or planes of oxygen atoms in open hexagonal rings. Depending on external pressure and temperature, water can freeze into eighteen different forms of ice.
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Why does ice float on water?
Ice floats on water because it is less dense than liquid water. Water molecules in liquid form are loosely connected by hydrogen bonds. As water freezes, these molecules arrange themselves into a more rigid and open hexagonal lattice. This arrangement increases the space between the molecules, which reduces the density of ice to approximately 917 kilograms per cubic meter, less than liquid water's density of about 1,000 kilograms per cubic meter.
Because ice is less dense, it floats, forming an insulating layer on the surface of bodies of water. This layer helps regulate the temperature of the water below, which protects aquatic life in cold conditions.
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How do people and nature use ice in everyday life?
Ice, the solid form of water, plays a multifaceted role in both human activities and natural processes. People use dry ice for food preservation during shipping, industrial cleaning, medical sample preservation, and creating special effects like fog in entertainment. In nature, ice appears as glaciers, sea ice, and river ice, each with unique characteristics and ecological functions. Sea ice, for example, creates habitats for viruses, bacteria, algae, fungi, and protozoans, which form concentrated food sources for krill and other zooplankton.
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News •
ice, solid substance produced by the freezing of water vapour or liquid water. At temperatures below 0 °C (32 °F), water vapour develops into frost at ground level and snowflakes (each of which consists of a single ice crystal) in clouds. Below the same temperature, liquid water forms a solid, as, for example, river ice, sea ice, hail, and ice produced commercially or in household refrigerators.
Ice occurs on Earth’s continents and surface waters in a variety of forms. Most notable are the continental glaciers (ice sheets) that cover much of Antarctica and Greenland. Smaller masses of perennial ice called ice caps occupy parts of Arctic Canada and other high-latitude regions, and mountain glaciers occur in more restricted areas, such as mountain valleys and the flatlands below. Other occurrences of ice on land include the different types of ground ice associated with permafrost—that is, permanently frozen soil common to very cold regions. In the oceanic waters of the polar regions, icebergs occur when large masses of ice break off from glaciers or ice shelves and drift away. The freezing of seawater in these regions results in the formation of sheets of sea ice known as pack ice. During the winter months similar ice bodies form on lakes and rivers in many parts of the world. This article treats the structure and properties of ice in general. Ice in lakes and rivers, glaciers, icebergs, pack ice, and permafrost are treated separately in articles under their respective titles. For a detailed account of the widespread occurrences of glacial ice during Earth’s past, see the articles geochronology and climate. See also glacial landform for the effects of glaciation.
Structure
The water molecule
Ice is the solid state of water, a normally liquid substance that freezes to the solid state at temperatures of 0 °C (32 °F) or lower and expands to the gaseous state at temperatures of 100 °C (212 °F) or higher. Water is an extraordinary substance, anomalous in nearly all its physical and chemical properties and easily the most complex of all the familiar substances that are single-chemical compounds. Consisting of two atoms of hydrogen (H) and one atom of oxygen (O), the water molecule has the chemical formula H2O. These three atoms are covalently bonded (i.e., their nuclei are linked by attraction to shared electrons) and form a specific structure, with the oxygen atom located between the two hydrogen atoms. The three atoms do not lie in a straight line, however. Instead, the hydrogen atoms are bent toward each other, forming an angle of about 105°.
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The three-dimensional structure of the water molecule can be pictured as a tetrahedron with an oxygen nucleus centre and four legs of high electron probability. The two legs in which the hydrogen nuclei are present are called bonding orbitals. Opposite the bonding orbitals and directed to the opposite corners of the tetrahedron are two legs of negative electrical charge. Known as the lone-pair orbitals, these are the keys to water’s peculiar behaviour, in that they attract the hydrogen nuclei of adjacent water molecules to form what are called hydrogen bonds. These bonds are not especially strong, but, because they orient the water molecules into a specific configuration, they significantly affect the properties of water in its solid, liquid, and gaseous states.
In the liquid state, most water molecules are associated in a polymeric structure—that is, chains of molecules connected by weak hydrogen bonds. Under the influence of thermal agitation, there is a constant breaking and reforming of these bonds. In the gaseous state, whether steam or water vapour, water molecules are largely independent of one another, and, apart from collisions, interactions between them are slight. Gaseous water, then, is largely monomeric—i.e., consisting of single molecules—although there occasionally occur dimers (a union of two molecules) and even some trimers (a combination of three molecules). In the solid state, at the other extreme, water molecules interact with one another strongly enough to form an ordered crystalline structure, with each oxygen atom collecting the four nearest of its neighbours and arranging them about itself in a rigid lattice. This structure results in a more open assembly, and hence a lower density, than the closely packed assembly of molecules in the liquid phase. For this reason, water is one of the few substances that is actually less dense in solid form than in the liquid state, dropping from 1,000 to 917 kilograms per cubic metre. It is the reason why ice floats rather than sinking, so that, during the winter, it develops as a sheet on the surface of lakes and rivers rather than sinking below the surface and accumulating from the bottom.
As water is warmed from the freezing point of 0 to 4 °C (from 32 to 39 °F), it contracts and becomes denser. This initial increase in density takes place because at 0 °C a portion of the water consists of open-structured molecular arrangements similar to those of ice crystals. As the temperature increases, these structures break down and reduce their volume to that of the more closely packed polymeric structures of the liquid state. With further warming beyond 4 °C, the water begins to expand in volume, along with the usual increase in intermolecular vibrations caused by thermal energy.