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water
Table of Contents
water

 * Introduction & Top Questions
   
 * Structure of water
    * Liquid water
   
    * Structures of ice
   
    * Significance of the structure of liquid water

 * Behaviour and properties
    * Water at high temperatures and pressures
   
    * Physical properties
   
    * Chemical properties
      * Acid-base reactions
      * Oxidation-reduction reactions

Fast Facts
 * 2-Min Summary
 * Top Questions
    * Where does water come from?
    * Why do cold water bottles and soft-drink bottles sweat?
    * When does water boil?
    * When is water the most dense?

 * Facts & Related Content

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Home Science Chemistry


WATER

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Alternate titles: H2O
By Steven S. Zumdahl • Edit History

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Table of Contents
water rapids, Niagara Falls, Canada
See all media
Key People: Jacques Dubochet Antoine Lavoisier Henry Cavendish Thales of Miletus
Charles-François Sturm ...(Show more) Related Topics: ice steam hydrate hard
water heavy water ...(Show more)
See all related content →
Top Questions
WHERE DOES WATER COME FROM?

Water is made up of hydrogen and oxygen, and it exists in gaseous, liquid, and
solid states. Water is one of the most plentiful and essential compounds,
occurring as a liquid on Earth’s surface under normal conditions, which makes it
invaluable for human uses and as plant and animal habitat. Since water is
readily changed to a vapour (gas), it can travel through the atmosphere from the
oceans inland, where it condenses and nourishes life.

WHY DO COLD WATER BOTTLES AND SOFT-DRINK BOTTLES SWEAT?

A cold water bottle appears to sweat because it’s a cooling source for the water
vapour in the layer of air that surrounds the bottle. Air that is relatively
warm can hold more water vapour than cooler air.  When the cold water bottle is
introduced, the warm air near the bottle cools and some of the water vapour
condenses into liquid water, which is then deposited on the outside of the
bottle.

WHEN DOES WATER BOIL?

Boiling occurs when bubbles form within a liquid, marking a change from a
substance’s liquid or solid phase into a gas. The normal boiling point is the
temperature at which the liquid’s vapour pressure equals the standard sea level
atmospheric pressure (760 mm [29.92 inches] of mercury). At sea level,
atmospheric pressure is high, and water boils at 100 °C (212 °F); at higher
altitudes it is lower, so water boils at a lower temperature.

WHY IS WATER BLUE?

Water appears blue for two important reasons. In small quantities water appears
colourless, but water actually has an intrinsic blue colour caused by the slight
absorption of light at red wavelengths. For larger bodies of water—ponds,
rivers, lakes, and oceans—water appears blue on clear days because it mirrors
the blueness of the sky. On overcast days, larger water bodies appear gray.

WHEN IS WATER THE MOST DENSE?

Water’s density is greatest at about 4 °C (39.2 °F), in the liquid phase. Ice,
water’s solid phase, is more buoyant, so it forms at the surface of water bodies
and freezes downward. Lakes and rivers rarely freeze completely, and the liquid
water below can become a winter refuge for aquatic life. When ice melts in the
spring, the slowly warming surface meltwater sinks, displacing the water below
and mixing nutrients throughout the water column.



water, a substance composed of the chemical elements hydrogen and oxygen and
existing in gaseous, liquid, and solid states. It is one of the most plentiful
and essential of compounds. A tasteless and odourless liquid at room
temperature, it has the important ability to dissolve many other substances.
Indeed, the versatility of water as a solvent is essential to living organisms.
Life is believed to have originated in the aqueous solutions of the world’s
oceans, and living organisms depend on aqueous solutions, such as blood and
digestive juices, for biological processes. Water also exists on other planets
and moons both within and beyond the solar system. In small quantities water
appears colourless, but water actually has an intrinsic blue colour caused by
slight absorption of light at red wavelengths.

chemistry of water
An overview of the chemical structure of water molecules.
© American Chemical Society (A Britannica Publishing Partner)See all videos for
this article

Although the molecules of water are simple in structure (H2O), the physical and
chemical properties of the compound are extraordinarily complicated, and they
are not typical of most substances found on Earth. For example, although the
sight of ice cubes floating in a glass of ice water is commonplace, such
behaviour is unusual for chemical entities. For almost every other compound, the
solid state is denser than the liquid state; thus, the solid would sink to the
bottom of the liquid. The fact that ice floats on water is exceedingly important
in the natural world, because the ice that forms on ponds and lakes in cold
areas of the world acts as an insulating barrier that protects the aquatic life
below. If ice were denser than liquid water, ice forming on a pond would sink,
thereby exposing more water to the cold temperature. Thus, the pond would
eventually freeze throughout, killing all the life-forms present.



polar bear on ice floe
Polar bear on an ice floe in the Svalbard archipelago. The sight of ice floating
on water is commonplace, but it shows the unusual chemical behaviour of water,
which is less dense as a solid than in its liquid phase.
© SeppFriedhuber—E+/Getty Images

Water occurs as a liquid on the surface of Earth under normal conditions, which
makes it invaluable for transportation, for recreation, and as a habitat for a
myriad of plants and animals. The fact that water is readily changed to a vapour
(gas) allows it to be transported through the atmosphere from the oceans to
inland areas where it condenses and, as rain, nourishes plant and animal life.
(See hydrosphere: The hydrologic cycle for a description of the cycle by which
water is transferred over Earth.)


diagram of the hydrologic cycle of water
In the hydrologic cycle, water is transferred between the land surface, the
ocean, and the atmosphere. The numbers on the arrows indicate relative water
fluxes.
Encyclopædia Britannica, Inc.

Because of its prominence, water has long played an important religious and
philosophical role in human history. In the 6th century bce, Thales of Miletus,
sometimes credited for initiating Greek philosophy, regarded water as the sole
fundamental building block of matter:



> It is water that, in taking different forms, constitutes the earth,
> atmosphere, sky, mountains, gods and men, beasts and birds, grass and trees,
> and animals down to worms, flies and ants. All these are different forms of
> water. Meditate on water!

Two hundred years later, Aristotle considered water to be one of four
fundamental elements, in addition to earth, air, and fire. The belief that water
was a fundamental substance persisted for more than 2,000 years until
experiments in the second half of the 18th century showed that water is a
compound made up of the elements hydrogen and oxygen.

water harvesting
Researchers invent a device that can harvest drinkable water from desert air.
Displayed by permission of The Regents of the University of California. All
rights reserved. (A Britannica Publishing Partner)See all videos for this
article

The water on the surface of Earth is found mainly in its oceans (97.25 percent)
and polar ice caps and glaciers (2.05 percent), with the balance in freshwater
lakes, rivers, and groundwater. As Earth’s population grows and the demand for
fresh water increases, water purification and recycling become increasingly
important. Interestingly, the purity requirements of water for industrial use
often exceed those for human consumption. For example, the water used in
high-pressure boilers must be at least 99.999998 percent pure. Because seawater
contains large quantities of dissolved salts, it must be desalinated for most
uses, including human consumption.


Hoover Dam
The Hoover Dam on the Colorado River at the border of Nevada and Arizona
demonstrates how natural resources of water can be harnessed for a variety of
purposes, including human consumption, irrigation, and industry.
Robert Glusic/Getty Images

diagram of a water treatment system
Water treatment systems are important for desalinating seawater so it can be
used for human consumption and for purifying water for industrial use.
Encyclopædia Britannica, Inc.

This article describes the molecular structure of water as well as its physical
and chemical properties. For other major treatments of water, see climate;
environmental works; hydrosphere; ice; and pollution.




STRUCTURE OF WATER




LIQUID WATER

The water molecule is composed of two hydrogen atoms, each linked by a single
chemical bond to an oxygen atom. Most hydrogen atoms have a nucleus consisting
solely of a proton. Two isotopic forms, deuterium and tritium, in which the
atomic nuclei also contain one and two neutrons, respectively, are found to a
small degree in water. Deuterium oxide (D2O), called heavy water, is important
in chemical research and is also used as a neutron moderator in some nuclear
reactors.



water molecule
A water molecule is made up of two hydrogen atoms and one oxygen atom. A single
oxygen atom contains six electrons in its outer shell, which can hold a total of
eight electrons. When two hydrogen atoms are bound to an oxygen atom, the outer
electron shell of oxygen is filled.
Encyclopædia Britannica, Inc.
Witness an experiment explaining why fresh water and seawater have different
freezing points
Learn why fresh water and seawater have different freezing points.
Contunico © ZDF Enterprises GmbH, MainzSee all videos for this article

Although its formula (H2O) seems simple, water exhibits very complex chemical
and physical properties. For example, its melting point, 0 °C (32 °F), and
boiling point, 100 °C (212 °F), are much higher than would be expected by
comparison with analogous compounds, such as hydrogen sulfide and ammonia. In
its solid form, ice, water is less dense than when it is liquid, another unusual
property. The root of these anomalies lies in the electronic structure of the
water molecule.



The water molecule is not linear but bent in a special way. The two hydrogen
atoms are bound to the oxygen atom at an angle of 104.5°.

The O―H distance (bond length) is 95.7 picometres (9.57 × 10−11 metres, or 3.77
× 10−9 inches). Because an oxygen atom has a greater electronegativity than a
hydrogen atom, the O―H bonds in the water molecule are polar, with the oxygen
bearing a partial negative charge (δ−) and the hydrogens having a partial
positive charge (δ+).

Hydrogen atoms in water molecules are attracted to regions of high electron
density and can form weak linkages, called hydrogen bonds, with those regions.
This means that the hydrogen atoms in one water molecule are attracted to the
nonbonding electron pairs of the oxygen atom on an adjacent water molecule. The
structure of liquid water is believed to consist of aggregates of water
molecules that form and re-form continually. This short-range order, as it is
called, accounts for other unusual properties of water, such as its high
viscosity and surface tension.


water droplets
Water is a polar molecule and is attracted to other polar molecules. Thus,
droplets, or beads, of water form on a nonpolar surface because water molecules
adhere together instead of adhering to the surface.
© fotofuerst/Fotolia
splitting water
A catalyst that splits water into hydrogen and oxygen.
© American Chemical Society (A Britannica Publishing Partner)See all videos for
this article

An oxygen atom has six electrons in its outer (valence) shell, which can hold a
total of eight electrons. When an oxygen atom forms a single chemical bond, it
shares one of its own electrons with the nucleus of another atom and receives in
return a share of an electron from that atom. When bonded to two hydrogen atoms,
the outer electron shell of the oxygen atom is filled.



The electron arrangement in the water molecule can be represented as follows.

Each pair of dots represents a pair of unshared electrons (i.e., the electrons
reside on only the oxygen atom). This situation can also be depicted by placing
the water molecule in a cube.

Each ↑↓ symbol represents a pair of unshared electrons. This electronic
structure leads to hydrogen bonding.



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External Websites
 * USGS Water Science School - Wastewater Treatment Water Use
 * The Chemistry LibreTexts Library - All About Water

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 * USGS Water Science School - Wastewater Treatment Water Use
 * The Chemistry LibreTexts Library - All About Water

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 * water - Children's Encyclopedia (Ages 8-11)
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