Water Definition in Chemistry

Of all the molecules in the universe, the one most important to humanity is water.

Water Definition

Water is a chemical compound consisting of two hydrogen atoms and one oxygen atom. The name water typically refers to the liquid state of the compound. The solid phase is known as ice and the gas phase is called steam. Under certain conditions, water also forms a supercritical fluid.

Other Names for Water

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Water Properties and Facts You Should Know

By Anne Marie Helmenstine, Ph.D.

The IUPAC name for water is, actually, water. The alternative name is oxidane. The name oxidane is only used in chemistry as the mononuclear parent hydride to name derivatives of water.

Other names for water include:

• Dihydrogen monoxide or DHMO
• Hydrogen hydroxide (HH or HOH)
• H₂O
• Hydrogen monoxide
• Dihydrogen oxide
• Hydric acid
• Hydrohydroxic acid
• Hydrol
• Hydrogen oxide
• The polarized form of water, H⁺ OH^-, is called hydron hyroxide.

The word "water" comes from the Old English word wæter or from the Proto-Germanic watar or German Wasser. All of these words mean "water" or "wet."

Important Water Facts

• Water is the main compound found in living organisms. Approximately 62 percent of the human body is water.
• In its liquid form, water is transparent and nearly colorless. Large volumes of liquid water and ice are blue. The reason for the blue color is the weak absorption of light at the red end of the visible spectrum.
• Pure water is flavorless and odorless.
• About 71 percent of the Earth's surface is covered by water. Breaking it down, 96.5 percent of the water in the Earth's crust is found in oceans, 1.7 percent in ice caps and glaciers, 1.7 percent in groundwater, a small fraction in rivers and lakes, and 0.001 percent in clouds, water vapor, and precipitation.
• Only about 2.5 percent of the Earth's water is freshwater. Nearly all of that water (98.8 percent) is in ice and groundwater.
• Water is the third most abundant molecule in the universe, after hydrogen gas (H₂) and carbon monoxide (CO).
• The chemical bonds between hydrogen and oxygen atoms in a water molecule are polar covalent bonds. Water readily forms hydrogen bonds with other water molecules. One water molecule may participate in a maximum of four hydrogen bonds with other species.
• Water has an extraordinarily high specific heat capacity [4.1814 J/(g·K) at 25 degrees C] and also a high heat of vaporization [40.65 kJ/mol or 2257 kJ/kg at the normal boiling point]. Both of these properties are a result of hydrogen bonding between neighboring water molecules.
• Water is nearly transparent to visible light and the regions of the ultraviolet and infrared spectrum near the visible range. The molecule absorbs infrared light, ultraviolet light, and microwave radiation.
• Water is an excellent solvent because of its polarity and high dielectric constant. Polar and ionic substances dissolve well in water, including acids, alcohols, and many salts.
• Water displays capillary action because of its strong adhesive and cohesive forces.
• Hydrogen bonding between water molecules also gives it high surface tension. This is the reason why small animals and insects can walk on water.
• Pure water is an electrical insulator. However, even deionized water contains ions because water undergoes auto-ionization. Most water contains trace amounts of solute. Often the solute is salt, which dissociates into ions and increases the conductivity of water.
• The density of water is about one gram per cubic centimeter. Regular ice is less dense than water and floats on it. Very few other substances exhibit this behavior. Paraffin and silica are other examples of substances that form lighter solids than liquids.
• The molar mass of water is 18.01528 g/mol.
• The melting point of water is 0.00 degrees C (32.00 degrees F; 273.15 K). Note the melting and freezing points of water may be different from each other. Water readily undergoes supercooling. It can remain in a liquid state well below its melting point.
• The boiling point of water is 99.98 degrees C (211.96 degrees F; 373.13 K).
• Water is amphoteric. In other words, it can act as both an acid and as a base.

Sources

• Braun, Charles L. "Why is water blue?" Journal of Chemical Education, Sergei N. Smirnov, ACS Publications, 1 August 1993.
• Gleick, Peter H. (Editor). "Water in Crisis: A Guide to the World's Freshwater Resources." Paperback, Oxford University Press, 26 August 1993.
• "Water." NIST Standard Reference Data, U.S. Secretary of Commerce on behalf of the United States of America, 2018.