Water | Research & Encyclopedia Articles

Water

Overview

Water is a colorless, odorless, tasteless liquid that also occurs commonly in the solid state (as ice) and in the gaseous state (as steam or water vapor). It is a very stable compound that undergoes a number of important reactions. It reacts with some metals to form elemental hydrogen and an inorganic base. With some metals, such as sodium and potassium, the reaction is quite violent. With other metals, such as iron, the reaction occurs only very slowly. Water is also a weak electrolyte, ionizing to form hydrogen ions (H⁺) and hydroxide ions (OH⁻). The hydrogen ions occur in solution as hydronium ions (H₃O⁺).

Key Facts

Other Names:

Hydrogen oxide; dihydrogen oxide

Formula:

H₂O or HOH

Elements:

Hydrogen, oxygen

Compound Type:

Inorganic

State:

Liquid

Molecular Weight:

18.02 g/mol

Melting Point:

0°C (32°F)

Boiling Point:

100°C (212°F)

Solubility:

Soluble in ethyl alcohol, methyl alcohol, and acetone

Water is also a strong dipole. A dipole is a molecule in which electrical charges are sufficiently separated from each other that one part of the molecule is negatively charged and another part, positively charged. Water's dipole character is responsible for many of its special characteristics. The positive end of one water molecule is attracted to the negative end of a second water molecule, resulting in the formation of a weak ("hydrogen") bond between the two molecules.

For example, water has a very high boiling point for a substance with relatively small molecules. The high boiling point is a result of the fact that heat added to water must first be used to break hydrogen bonds between water molecules before providing enough energy to vaporize the molecules. Similarly, the phenomenon known as surface tension is caused by hydrogen bonding. Surface tension is the tendency of a liquid to act as if it is covered with a thin film. Some insects are able to walk on water because its surface tension is so great. The surface tension is caused by the attractive forces between adjacent water molecules.

Water is also an excellent solvent. A solvent is a substance capable of dissolving other substances. Chemists sometimes refer to water as "the universal solvent" because it is able to dissolve so many other substances. That statement is an exaggeration, but does reflect the compound's ability to dissolve more substances that probably any other single compound. Water's ability to dissolve other sub stances is at least partly a result of its strong dipole character. The positive or negative end of a water molecule attaches itself to the negative or positive end of the substance to be dissolved. The force of attraction exerted by the water molecule is sufficient to tear apart the particles of which the second substance is composed causing them to dissolve in the water.

How It Is Made

Water can be made by a variety of chemical reactions, including:

• The oxidation of hydrogen: 2H₂ + O₂ → 2H₂O;
• The reaction between an acid and a base, as, for example: NaOH + HCl → NaCl + H₂O;
• The combustion of an organic material, as, for example: CH₄ + 2O₂ → CO₂ + 2H₂O.

Interesting Facts

• When water freezes, the attractive forces between molecules force them into a regular crystalline pattern that occupies more space than do the same water molecules in the liquid state. In other words, water expands when it freezes and ice is less dense than liquid water.
• One consequence of that fact is that lakes freeze from the top down. As ice forms, it floats to the surface of the liquid. Aquatic organisms in the lake are able to live through the winter because of the layer of ice on top of the lake.
• Adding to the protection added by the layer of ice is the fact that ice is one of the best thermal (heat) insulators known. That is, heat flows through ice more slowly than through almost any other substance. Any heat left in a freezing lake will be conserved within the water since it escapes so slowly through the ice layer.

Because water occurs so abundantly, none of these reactions is required for the commercial production of the compound. Water makes up about 70 percent of the Earth's surface in the oceans, lakes, rivers, ponds, glaciers, ice caps, and other reservoirs. The problem is that only a very small fraction of that water—about 3 percent—is fresh water. The remaining 97 percent is salt water. And even the 3 percent of fresh water in lakes, rivers, and other resources is impure, in the sense that it contains other substances dissolved and suspended in it.

Thus, the primary concern in obtaining adequate supplies of pure water for household, personal, commercial, industrial, or other uses is the purification of water, not its synthesis. Purification of water is achieved by a number of processes, including chlorination, filtration, distillation, or purification by some type of ion exchange mechanism.

Common Uses and Potential Hazards

One of the most important uses of water is the survival of life on Earth. All plants and animals contain a high proportion of water. That proportion ranges from as high as 97 percent in many fruits and vegetables to a low of about 20 percent in some "dry" foods like breads and cereals. The human body itself comprises anywhere from 50 to 70 percent of a person's body weight. That water plays a number of roles, such as providing a solvent by which nutrients are circulated throughout the body and making possible all kinds of chemical reactions that occur in aqueous solutions, but do not occur in the dry state.

Although easy to ignore, water plays an almost unlimited number of roles in industrial, chemical, commercial, and other operations. Among these applications of water are:

• As a coolant in electricity generating plants, petroleum refineries, chemical plants, and many kinds of industrial factories;
• For irrigation;
• For cleaning, washing, and scouring raw materials and finished products;
• As a source of hydrogen and oxygen for many industrial and chemical operations;
• As a solvent for many types of industrial and chemical reactions and for the extraction of compounds from mixtures;
• In the manufacture of many kinds of foods and beverages, such as beer, wine, and soft drinks;
• As a medium for suspending materials in industrial processes, such as the manufacture of paper;
• In the processing of textiles;
• For the generation of steam to power industrial, household, and chemical processes;

• For the hydration of lime;
• As a coolant in nuclear reactors;
• In the transport of industrial and chemical raw materials and products;
• For the removal of barks from logs in the timber industry;
• To make possible hydrolysis reactions in chemical and industrial operations;
• In the manufacture of Portland cement; and
• To dilute solutions that are too concentrated for some given industrial process.

Words to Know

AQUEOUS SOLUTION

A solution that consists of some material dissolved in water.

ELECTROLYTE

A substance which, when dissolved in water, will conduct an electric current.

HYDROLYSIS

The process by which a compound reacts with water to form two new compounds.

For Further Information

"Iowa Project WET." Iowa Academy of Science. http://www.uni.edu/∼iowawet/iowawet.html (accessed on November 19, 2005).

Strange, Veronica. The Meaning of Water. Oxford, UK: Berg Publishers, 2004.

"Water Properties." London South Bank University. http://www.lsbu.ac.uk/water/data.html (accessed on November 19, 2005).

"Water Science for Schools." U.S. Geological Survey. http://ga.water.usgs.gov/edu/ (accessed on November 19, 2005).

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