Water and ice

Water is a molecule formed from two hydrogen atoms covalently bonded to a single oxygen atom. The chemical properties of water are unique among molecular substances on Earth and have had a dominant effect on the evolution of life.

Chemical Properties of Water

Water (H₂O), also called dihydrogen monoxide, is a naturally occurring molecule that can exist on Earth in solid, liquid, and gaseous phases. Water is the most abundant molecule on Earth, covering approximately 70 percent of the earth’s surface.

The hydrosphere, which is composed of the collective waters of the seas, oceans, rivers, lakes, and the lower atmosphere, consists of more than 1.3 billion cubic kilometers (312 million cubic miles) of water. The chemical properties of the water molecule are in large part responsible for the nature of life on Earth. The bodies of most living organisms are more than 60 percent water, and some living cells contain more than 90 percent water by volume.

The molecular structure of water consists of a single oxygen atom joined with two hydrogen atoms through covalent bonding, which involves the sharing of electrons between two atoms. Water molecules are often described as having a bent shape, because the pull of oxygen on its shared electrons is stronger than the pull of hydrogen. For this reason, the hydrogen atoms tend to gather toward one side of the water molecule, while lone electrons are pulled to the other side. This chemical symmetry means that one side of the molecule has a slight negative charge and the other (the side with the hydrogen atoms) has a slight positive charge. Water is therefore a polar molecule, which is attracted to other water molecules and to other molecules and ions that have a positive or negative charge.

Because of their slight positive and negative charges, water molecules bond to one another through hydrogen bonding, which is the momentary attraction of the negative end of one water molecule with the positive end of a nearby molecule. In liquid water, molecules are constantly moving, and the hydrogen bonds between them are frequently broken and re-formed. The average hydrogen bond lasts for less than one trillionth of a second before being broken. However, the millions of hydrogen bonds forming across a body of water provide a collective strength that gives water unique structural and adhesive properties.

The polarity of water molecules makes water an excellent solvent, meaning it can cause the dissolution of other molecules and substances, as water molecules form bonds with other polar molecules they encounter. Any compound with a negative or positive net charge will dissolve in liquid water. Water’s function as a solvent is important in a variety of biological processes and also allows water to dissolve minerals and other elements from the lithosphere of the earth, returning them to the hydrosphere and biosphere and thereby acting as the most important catalyst in many geochemical cycles.

Hydrogen bonding gives water high surface tension, which is the capacity of the layer on the surface of a liquid to resist external pressure. Therefore, certain substances and materials will float on top of a body of water rather than penetrate the surface. In addition, hydrogen bonds cause water to tend to gather into drops, rather than spreading evenly, which is important to plants that use water’s natural adhesion to help distribute water throughout the plant’s tissues.

The collective energy of hydrogen bonding also means that water has a high specific heat, which is the heat at which the liquid form will give rise to a gas. Water therefore must absorb a large amount of heat energy before it will change temperature, and it also tends to release heat energy slowly back into the environment. These properties mean that water is a natural regulator of climate, tending to absorb excess heat when available and to release heat later when the ambient heat has reduced.

Properties of Ice

Most substances become more dense as they cool because heat energy is lost, causing collisions between molecules to decrease and thereby allowing molecules to settle closer to one another. As water cools, its density increases until the temperature reaches 4 degrees Celsius (39 degrees Fahrenheit), reaching the maximum density for liquid water. If water continues to cool beyond this point, the vibrational energy of individual water molecules decreases to such a point that the hydrogen bonds between molecules become fixed, rather than alternate between bonds. At this point, water begins to decrease in density as the molecules align into a matrix, simultaneously increasing in volume.

At approximately 0 degrees Celsius (32 degrees Fahrenheit) liquid water becomes solid ice. The organized arrangement of atoms in ice allow for more space between molecules, thereby increasing the volume by more than 9 percent. This property is unusual among liquids, but it also occurs in the common mineral silica (SiO₂) as it transitions from liquid magma to solid silica rock. Because ice is less dense than water, it does not sink and will rise to the surface of the liquid, further buoyed by the surface tension of the liquid. In areas where freezing occurs, this creates a layer of frozen ice that insulates the liquid water beneath. This insulation prevents further water from reaching the freezing point and thereby allows organisms to survive under the surface of frozen bodies of water. Similarly, the preservation of this liquid water means that chemical reactions can continue under the surface of the ice.

The

The hydrologic or water cycle is a geochemical cycle that moves water molecules between the environmental spheres of the earth. The lithosphere is the combination of rocky minerals, sediment, and molten rock that make up the earth’s crust and part of the earth’s mantle.

The atmosphere is an envelope of gaseous elements that surround the earth in layers and differentiate the planetary environment from that of outer space. The biosphere is the sum of all living organisms on the planet and their interactions with the other environmental spheres. Geochemical cycles are the processes that move various elements through the environmental spheres through interconnected sets of chemical and physical reactions.

The hydrologic cycle begins with evaporation, which is the process by which liquid water transitions into a gaseous form and enters the earth’s atmosphere. Approximately 80 percent of evaporation occurs over the ocean, with the remainder occurring over lakes, rivers, and water temporarily deposited on the lithosphere. Water rises into the atmosphere as water vapor and, as it gains in altitude, the temperature of the atmosphere decreases, causing water vapor to condense. Hydrogen bonds cause water vapor to form into clouds, which are pockets of water droplets or ice crystals that move according to the development of thermal currents in the atmosphere.

Water vapor in clouds and in more diffuse gaseous form throughout the atmosphere is transported around the earth on thermal currents and eventually returns to the earth as precipitation, which is the return of condensed water to the hydrosphere and lithosphere in the form of rain and snow. Precipitation occurs when the density of water vapor in the atmosphere reaches a certain critical level, often triggered by temperature changes in the atmosphere that are related to windstorms that develop over the ocean.

A portion of the water delivered to the lithosphere as precipitation returns to the atmosphere through evaporation, because the temperature of the lithosphere is generally warmer than the temperature of the hydrosphere; this causes water to more easily vaporize into gas. Some of the precipitation over the lithosphere penetrates the minerals of the earth and becomes groundwater, which is water that saturates and remains within the mineral layers of the earth. Flooding occurs when groundwater levels rise to a point that the soil is oversaturated and can no longer absorb additional water.

Groundwater eventually returns to the hydrosphere as it filters into streams and lakes that are connected to the ocean. Additional groundwater enters the ocean as seepage in coastal areas, where the lithosphere and the ocean meet. Water that filters back into the hydrosphere through the lithosphere carries a variety of minerals and other trace elements because of the solvent properties of water. These minerals enrich water sources, thereby contributing to the nutrient cycle in aquatic environments.

The organisms of the biosphere also absorb water into their tissues, using it to fuel and to carry the products of chemical reactions. Water from within organisms is returned to the atmosphere during respiration and as a component of biological waste. Water returned to the hydrosphere through the biosphere also carries a variety of minerals and other elements that are thereafter transferred through the hydrosphere into the other environmental spheres.

Hydroelectricity

Hydroelectricity is the process of using water to generate electricity for human consumption. The basic method is to harness the kinetic energy of moving water and then to transition this energy into electric currents that can be stored and used to power electric devices.

Generally, the generation of hydroelectricity involves using running water to power turbine engines, devices with spinning axles attached to blades that move in response to the flow of a fluid through the blades. As the blades turn, they power an electric generator, which forces a flow of electrons through a circuit and generates electric currents.

Hydroelectric generators can utilize the gravitational flow of liquid water, which is the tendency for water to flow downward with the force of gravity from an area of higher elevation or depth. To harness this energy, engineers control the flow of water and filter the water through channels containing turbines.

A number of different methods generate hydroelectricity, depending on environmental conditions. Hydroelectric dams function by blocking the flow of water along the path of a river or other waterway. This creates an artificially generated imbalance in water depth on either side of the dam. The higher water on one side of the dam can then be used to power the flow of water through the turbines at the bottom of the dam. Another way of artificially generating water flow is to utilize a series of pumps to control or enhance the flow of water in a certain area. In this case, hydroelectricity can be generated continuously, rather than relying on naturally occurring gravitational imbalances to generate flow.

Turbines can be installed in rivers to capture energy from the flow of the river current. Similarly, turbines can be installed in oceanic areas, where they derive energy from the tidal movement of ocean currents. Tidal power and wave power are two of the most recent forms of hydroelectricity and have developed into major areas of research. Energy present in the kinetic movement of tides provides a consistent and predictable source of energy generation but is utilized in relatively few areas.

All forms of hydroelectric energy offer the advantage of being renewable, as they are not based on the supply of a limited ingredient. However, this type of energy usage often carries environmental consequences because the environment must be altered to install hydroelectric generators and other equipment.

Principal Terms

covalent bond: powerful bond between two atoms that involves the sharing of a pair of electrons

evaporation: type of vaporization, the transition of a liquid to gaseous state; occurs on the top layer of a liquid

groundwater: water that penetrates the surface of the lithosphere and gathers in layers beneath the crust of the earth

hydroelectricity: methods used to convert the kinetic energy in moving water to electric energy

hydrogen bond: electrochemical bond between a hydrogen atom and an electropositive atom such as those that occur between water molecules in liquid water and ice

hydrosphere: the collective waters of the earth contained in the oceans, rivers, seas, lakes, and the lower level of the atmosphere

precipitation: rain, sleet, hail, snow, or other forms of liquid or solid water that fall from the atmosphere to the hydrosphere or lithosphere

solvent: substance that dissolves the chemical structure of another substance upon contact

specific heat: the heat at which a liquid will transition to a gaseous state

surface tension: measure of the potential for the surface of a liquid to resist external force

Bibliography

Barry, Roger, and Thian Yew Gan. The Global Cryosphere: Past, Present, and Future. New York: Cambridge University Press, 2011.

Brini, Emiliano, et al. "How Water’s Properties Are Encoded in Its Molecular Structure and Energies." Chemical Reviews, vol. 117, no. 19, 26 Sept. 2017, 12385–12414, doi: 10.1021/acs.chemrev.7b00259. Accessed 26 July 2024.

Brutsaert, Wilfried. Hydrology: An Introduction. New York: Cambridge University Press, 2005.

Gosnell, Mariana. Ice: The Nature, the History, and the Uses of an Astonishing Substance. New York: Alfred A. Knopf, 2005.

"The Hydrologic Cycle." National Oceanic and Atmospheric Administration, 24 Mar. 2023, www.noaa.gov/jetstream/atmosphere/hydro. Accessed 26 July 2024.

Jacobson, Michael C., et al. Earth System Science: From Biogeochemical Cycles to Global Change. San Diego, Calif.: Academic Press, 2006.

Macdougall, Douglas. Frozen Earth. Berkeley: University of California Press, 2006.

Tagare, Digamber M. Electricity Power Generation: The Changing Dimensions. Hoboken, N.J.: Wiley-Blackwell, 2011.

Wallace, John M., and Peter V. Hobbs. Atmospheric Science: An Introductory Survey. 2d ed. Burlington, Mass.: Academic Press, 2006.