Ice

Ice is the solid state of water. It typically forms when temperatures reach 0 °C, 32 °F, or 273.15 K.

Because it is a naturally occurring crystalline inorganic solid with an ordered structure, scientists classify ice as a mineral. It is found across the universe. It exists on Earth and other planets. It is also found in Oort cloud objects and as interstellar ice.

The appearance of ice varies. It can be transparent or a bluish-white color. This depends on impurities like air bubbles or soil particles.

To understand how ice works, we must look at its molecular structure. Water molecules consist of one oxygen atom covalently bonded to two hydrogen atoms, known as H–O–H.

These molecules are held together by hydrogen bonds. While these bonds are weak, they control the structure of both water and ice. As liquid water cools below 0 °C at standard atmospheric pressure, it undergoes a phase transition to ice Ih. This is a hexagonal crystalline structure.

During this process, hydrogen bonding becomes the dominant force. This causes the molecules to arrange into a less compact packing geometry than in liquid water.

This specific arrangement leads to an unusual physical property. Ice is approximately 8.3% less dense than liquid water. This results in a volumetric expansion of about 9% when fresh water freezes.

Liquid water is densest at 4 °C. As it cools further toward the freezing point, it loses density. Because ice is less dense, it floats on liquid water. This prevents bodies of water from freezing from the bottom up. A sheltered environment is created beneath the ice for plants and animals. This layer protects them from wind chill and weather extremes. In the ocean, sea ice contains brine-filled channels. These channels support organisms like algae, copepods, and annelids. These, in turn, provide food for larger animals like krill, fish, penguins, and whales.

Ice can also exist in many different phases. Scientists have identified at least nineteen known crystalline phases. These phases change based on temperature and pressure.

For example, Ice XII was discovered in 1996. In 2006, researchers found Ice XIII and Ice XIV. Ice XV was discovered in 2009 at extremely high pressures. There are also amorphous states, which lack a regular crystal structure. In the interstellar medium, low-density amorphous ice (LDA) is dominant. This may make LDA the most abundant type of ice in the universe. At extremely high pressures, ice is even predicted to become a metal.

The hardness of ice changes with its temperature. At its melting point, ice has a Mohs hardness of 2 or less. As it gets colder, it becomes much harder. At −20 °C, the hardness increases to about 4. By the time it reaches −78 °C, the vaporization point of dry ice, the hardness reaches 6. The color of ice is also affected by physics. Ice absorbs light at the red end of the spectrum. This makes ice appear blue, often with a greenish tint. This effect becomes more intense as the ice gets thicker.

Humans have interacted with ice for thousands of years. Some historic structures built to store ice for cooling are over 2,000 years old.

Before modern refrigeration, ice was the only way to store food safely without preservatives. Ice also has practical uses for transport. If the surface ice is solid enough, it creates ice roads for land transport. It is also essential for winter sports.

The low coefficient of friction makes ice slippery. While scientists once thought "pressure melting" caused this, modern research suggests other factors. One theory is that frictional heating is the most important process during movement.

Finally, ice is a critical part of Earth's systems. It is a major component of the water cycle and the climate. Ice can fall as precipitation, such as snow or hail. It can also form as frost through direct deposition from water vapor. The transition from ice to liquid is called melting. The transition from ice directly to vapor is called sublimation.

Recently, Earth's ice volume has been decreasing due to climate change. The largest declines are in the Arctic and in mountain regions. The loss of grounded ice is the primary cause of rising sea levels.