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Water in three states: liquid, solid (
ice), and (invisible)
water vapor in the air.
Clouds are accumulations of water droplets,
condensed from vapor-saturated air.
Water is a
chemical substance with the
chemical formula H2O. Its molecule contains one
oxygen and two
hydrogen atoms connected by
covalent bonds. Water is a
liquid at
ambient conditions, but it often co-exists on Earth with its
solid state,
ice, and
gaseous state (
water vapor or
steam). Water also exists in a
liquid crystal state near
hydrophilic surfaces.
[1][2]
Water covers 70.9% of the
Earth's surface,
[3] and is vital for all known forms of
life.
[4] On Earth, it is found mostly in oceans and other large water bodies, with 1.6% of water below ground in
aquifers and 0.001% in the
air as
vapor,
clouds (formed of solid and liquid water particles suspended in air), and
precipitation.
[5] Oceans hold 97% of surface water,
glaciers and polar
ice caps 2.4%, and other land surface water such as
rivers,
lakes and
ponds 0.6%. A very small amount of the Earth's water is contained within biological bodies and manufactured products.
Water on Earth moves continually through a
cycle of
evaporation or
transpiration (
evapotranspiration),
precipitation, and
runoff, usually reaching the
sea. Over land, evaporation and transpiration contribute to the precipitation over land.
Clean
drinking water is essential to
humans and other lifeforms. Access to safe drinking water has improved steadily and substantially over the last decades in almost every part of the world.
[6][7] There is a clear correlation between access to safe water and
GDP per capita.
[8] However, some observers have estimated that by 2025 more than half of the
world population will be facing water-based vulnerability.
[9] A recent report (November 2009) suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%.
[10] Water plays an important role in the
world economy, as it functions as a
solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of
freshwater is consumed by
agriculture.
[11]
Chemical and physical properties
Impact from a water drop causes an upward "rebound" jet surrounded by circular
capillary waves.
Water is the
chemical substance with
chemical formula H
2O: one
molecule of water has two
hydrogen atoms covalently bonded to a single
oxygen atom.
Water appears in nature in all three common states of matter and may take many different forms on Earth: water vapor and clouds in the sky;
seawater and
icebergs in the polar oceans;
glaciers and
rivers in the
mountains; and the liquid in aquifers in the ground.
At high temperatures and pressures, such as in the interior of giant planets, it is argued that water exists as
ionic water in which the molecules break down into a soup of hydrogen and oxygen ions, and at even higher pressures as
superionic water in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice.
[12]
The major chemical and physical properties of water are:
- Since the water molecule is not linear and the oxygen atom has a higher electronegativity than hydrogen atoms, it carries a slight negative charge, whereas the hydrogen atoms are slightly positive. As a result, water is a polar molecule with an electrical dipole moment. Water also can form an unusually large number of intermolecular hydrogen bonds (four) for a molecule of its size. These factors lead to strong attractive forces between molecules of water, giving rise to water's high surface tension[14] and capillary forces. The capillary action refers to the tendency of water to move up a narrow tube against the force of gravity. This property is relied upon by all vascular plants, such as trees.[citation needed]
- Water is a good solvent and is often[quantify] referred to[by whom?] as the universal solvent. Substances that dissolve in water, e.g., salts, sugars, acids, alkalis, and some gases – especially oxygen, carbon dioxide (carbonation) are known as hydrophilic (water-loving) substances, while those that do not mix well with water (e.g., fats and oils), are known as hydrophobic (water-fearing) substances.
- The boiling point of water (and all other liquids) is dependent on the barometric pressure. For example, on the top of Mt. Everest water boils at 68 °C (154 °F), compared to 100 °C (212 °F) at sea level. Conversely, water deep in the ocean near geothermal vents can reach temperatures of hundreds of degrees and remain liquid.
- Water has the second highest molar specific heat capacity of any known substance, after ammonia, as well as a high heat of vaporization (40.65 kJ·mol−1), both of which are a result of the extensive hydrogen bonding between its molecules. These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature.
- The maximum density of water occurs at 3.98 °C (39.16 °F).[15] It has the anomalous property of becoming less dense, not more, when it is cooled down to its solid form, ice. It expands to occupy 9% greater volume in this solid state, which accounts for the fact of ice floating on liquid water.
- Its Density is 1,000 kg/m3 liquid (4 °C), and weighs 62.4 lb/ft.3 (917 kg/m3, solid). It weighs 8.3454 lb/gal. (US, liquid) [16]
ADR label for transporting goods dangerously reactive with water
- Water is miscible with many liquids, such as ethanol, in all proportions, forming a single homogeneous liquid. On the other hand, water and most oils are immiscible usually forming layers according to increasing density from the top. As a gas, water vapor is completely miscible with air.
- Water forms an azeotrope with many other solvents.
- As an oxide of hydrogen, water is formed when hydrogen or hydrogen-containing compounds burn or react with oxygen or oxygen-containing compounds. Water is not a fuel, it is an end-product of the combustion of hydrogen. The energy required to split water into hydrogen and oxygen by electrolysis or any other means is greater than the energy that can be collected when the hydrogen and oxygen recombine.[17]
Taste and odor
Water can dissolve many different substances, giving it varying tastes and odors.
Humans and other animals have developed senses which enable them to evaluate the
potability of water by avoiding water that is too salty or
putrid. The taste of
spring water and
mineral water, often advertised in marketing of consumer products, derives from the minerals dissolved in it. However, pure H
2O is tasteless and odorless. The advertised purity of spring and mineral water refers to absence of
toxins,
pollutants and
microbes.
Distribution in nature
In the universe
Much of the universe's water is produced as a byproduct of
star formation. When stars are born, their birth is accompanied by a strong outward wind of gas and dust. When this outflow of material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water observed is quickly produced in this warm dense gas.
[18]
Water has been detected in
interstellar clouds within our
galaxy, the
Milky Way. Water probably exists in abundance in other galaxies, too, because its components, hydrogen and oxygen, are among the most abundant elements in the universe. Interstellar clouds eventually condense into
solar nebulae and
solar systems such as ours.
Water vapor is present in
Liquid water is present on
Strong evidence suggests that liquid water is present just under the surface of Saturn's moon
Enceladus. Jupiter's moon
Europa may have liquid water in the form as a 100 km deep subsurface ocean, which would amount to more water than is in all the Earth's oceans.
Water ice is present on
Water ice may be present on
Ceres and
Tethys. Water and other
volatiles probably comprise much of the internal structures of
Uranus and
Neptune and the water in the deeper layers may be in the form of
ionic water in which the molecules break down into a soup of hydrogen and oxygen ions, and deeper down as
superionic water in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice.
[12]
Some of the Moon's minerals contain water molecules. For instance, in 2008 a laboratory device which ejects and identifies particles found small amounts of the compound in the inside of volcanic pearls brought from Moon to Earth by the
Apollo 15 crew in 1971.
[23] NASA reported the detection of water molecules by NASA's Moon Mineralogy Mapper aboard the Indian Space Research Organization's Chandrayaan-1 spacecraft in September 2009.
[24]
Water and habitable zone
The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth are vital to the existence of
life on Earth as we know it. The Earth is located in the
habitable zone of the
solar system; if it were slightly closer to or farther from the
Sun (about 5%, or about 8 million kilometers), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.
[25][26]
Earth's
gravity allows it to hold an
atmosphere. Water vapor and carbon dioxide in the atmosphere provide a temperature buffer (
greenhouse effect) which helps maintain a relatively steady surface temperature. If Earth were smaller, a thinner atmosphere would allow temperature extremes, thus preventing the accumulation of water except in
polar ice caps (as on
Mars).
The surface temperature of Earth has been relatively constant through
geologic time despite varying levels of incoming solar radiation (
insolation), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric
albedo. This proposal is known as the
Gaia hypothesis.
The state of water on a planet depends on ambient pressure, which is determined by the planet's gravity. If a planet is sufficiently massive, the water on it may be solid even at high temperatures, because of the high pressure caused by gravity, as it was observed on exoplanets
Gliese 436 b[27] and
GJ 1214 b.
[28]
There are various theories about
origin of water on Earth.
On Earth
A graphical distribution of the locations of water on Earth.
Water covers 71% of the Earth's surface; the oceans contain 97.2% of the Earth's water. The
Antarctic ice sheet, which contains 61% of all fresh water on Earth, is visible at the bottom. Condensed atmospheric water can be seen as
clouds, contributing to the Earth's
albedo.
Hydrology is the study of the movement, distribution, and quality of water throughout the Earth. The study of the distribution of water is
hydrography. The study of the distribution and movement of groundwater is
hydrogeology, of glaciers is
glaciology, of inland waters is
limnology and distribution of oceans is
oceanography. Ecological processes with hydrology are in focus of
ecohydrology.
The collective mass of water found on, under, and over the surface of a planet is called the
hydrosphere. Earth's approximate water volume (the total water supply of the world) is 1,360,000,000 km
3 (326,000,000 mi
3).
Groundwater and fresh water are useful or potentially useful to humans as
water resources.
Liquid water is found in
bodies of water, such as an ocean,
sea,
lake,
river,
stream,
canal,
pond, or
puddle. The majority of water on Earth is
sea water. Water is also present in the atmosphere in solid, liquid, and vapor states. It also exists as groundwater in
aquifers.
Water is important in many geological processes. Groundwater is present in most
rocks, and the pressure of this groundwater affects patterns of
faulting. Water in the
mantle is responsible for the melt that produces
volcanoes at
subduction zones. On the surface of the Earth, water is important in both chemical and physical
weathering processes. Water and, to a lesser but still significant extent, ice, are also responsible for a large amount of
sediment transport that occurs on the surface of the earth.
Deposition of transported sediment forms many types of
sedimentary rocks, which make up the
geologic record of
Earth history.
Water cycle
Main article:
Water cycleThe
water cycle (known scientifically as the
hydrologic cycle) refers to the continuous exchange of water within the
hydrosphere, between the
atmosphere,
soil water,
surface water,
groundwater, and
plants.
Water moves perpetually through each of these regions in the
water cycle consisting of following transfer processes:
- evaporation from oceans and other water bodies into the air and transpiration from land plants and animals into air.
- precipitation, from water vapor condensing from the air and falling to earth or ocean.
- runoff from the land usually reaching the sea.
Most water vapor over the oceans returns to the oceans, but winds carry water vapor over land at the same rate as runoff into the sea, about 36
Tt per year. Over land, evaporation and transpiration contribute another 71 Tt per year. Precipitation, at a rate of 107 Tt per year over land, has several forms: most commonly
rain,
snow, and
hail, with some contribution from
fog and
dew. Condensed water in the air may also
refract sunlight to produce
rainbows.
Water runoff often collects over
watersheds flowing into rivers. A mathematical model used to simulate river or stream flow and calculate water quality parameters is
hydrological transport model. Some of water is diverted to
irrigation for agriculture. Rivers and seas offer opportunity for
travel and
commerce. Through
erosion, runoff shapes the environment creating river
valleys and
deltas which provide rich soil and level ground for the establishment of population centers. A
flood occurs when an area of land, usually low-lying, is covered with water. It is when a river overflows its banks or flood from the sea. A
drought is an extended period of months or years when a region notes a deficiency in its water supply. This occurs when a region receives consistently below average precipitation.
Fresh water storage
High tide (left) and low tide (right)
Some runoff water is trapped for periods of time, for example in lakes. At high altitude, during winter, and in the far north and south, snow collects in ice caps, snow pack and glaciers. Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface in
springs, or more spectacularly in
hot springs and
geysers. Groundwater is also extracted artificially in
wells. This water storage is important, since clean, fresh water is essential to human and other land-based life. In many parts of the world, it is in short supply.
Sea water
Sea water contains about 3.5%
salt on average, plus smaller amounts of other substances. The physical properties of sea water differ from fresh water in some important respects. It freezes at a lower temperature (about −1.9 °C) and its density increases with decreasing temperature to the freezing point, instead of reaching maximum density at a temperature above freezing. The salinity of water in major seas varies from about 0.7% in the
Baltic Sea to 4.0% in the
Red Sea.
Tides
Tides are the cyclic rising and falling of local sea levels caused by the
tidal forces of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and
estuarine water bodies and produce oscillating currents known as tidal streams. The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the
effects of Earth rotation and the local
bathymetry. The strip of seashore that is submerged at high tide and exposed at low tide, the
intertidal zone, is an important ecological product of ocean tides.
Effects on life
Overview of
photosynthesis and
respiration. Water (at right), together with carbon dioxide (CO
2), form oxygen and organic compounds (at left), which can be respired to water and (CO
2).
From a
biological standpoint, water has many distinct properties that are critical for the proliferation of
life that set it apart from other substances. It carries out this role by allowing
organic compounds to react in ways that ultimately allow
replication. All known forms of life depend on water. Water is vital both as a
solvent in which many of the body's solutes dissolve and as an essential part of many
metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g. starches, triglycerides and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g. glucose, fatty acids and amino acids to be used for fuels for energy use or other purposes). Without water, these particular metabolic processes could not exist.
Water is fundamental to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. Hydrogen is combined with CO
2 (absorbed from air or water) to form glucose and release oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO
2 in the process (cellular respiration).
Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H
+, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as hydroxide ion (OH
−) to form water. Water is considered to be neutral, with a
pH (the negative log of the hydrogen ion concentration) of 7.
Acids have pH values less than 7 while
bases have values greater than 7.
Aquatic life forms
