Water: The Universal Solvent and solvent of life
Water (H2O) is
a polar inorganic compound that is at room temperature a tasteless and odorless
liquid, nearly colorless with a hint of blue.
This simplest hydrogen
chalcogenide is by far the most studied chemical compound and is described as
the "universal solvent" for its ability to dissolve many substances.
This allows it
to be the "solvent of life".
It is the only common substance to exist as a
solid, liquid, and gas in nature.
Water
molecules form hydrogen bonds with each other and are strongly polar.
This
polarity allows it to separate ions in salts and strongly bond to other polar
substances such as alcohols and acids, thus dissolving them.
Its hydrogen
bonding causes its many unique properties, such as having a solid form less
dense than its liquid form, a relatively high boiling point of 100 °C for its
molar mass, and a high heat capacity.
Water is
amphoteric, meaning it is both an acid and a base—it produces H+ and OH−
ions by self ionization. This regulates the
concentrations of H+ and OH− ions in water.
Due to water
being a very good solvent, it is rarely pure and some of the properties of
impure water can vary from those of the pure substance. However, there are also
many compounds that are essentially, if not completely, insoluble in water,
such as fats, oils and other non-polar substances.
The accepted
IUPAC name of water is oxidane or simply water,or its equivalent in
different languages, although there are other systematic names which can be
used to describe the molecule.
Oxidane is only intended to be used as the name
of the mononuclear parent hydride used for naming derivatives of water by
substituent nomenclature.
These derivatives commonly have other recommended
names. For example, the name hydroxyl is recommended over oxidanyl for the –OH
group.
The name oxane is explicitly mentioned by the IUPAC as being unsuitable
for this purpose, since it is already the name of a cyclic ether also known as
tetrahydropyran.
The simplest
systematic name of water is hydrogen oxide. This is analogous to related
compounds such as hydrogen peroxide, hydrogen sulfide, and deuterium oxide
(heavy water).
The polarized
form of the water molecule, H+ OH−, is also
called hydron hydroxide by IUPAC nomenclature.
In keeping
with the basic rules of chemical nomenclature, water would have a
systematic name of dihydrogen monoxide, but this is not among the names
published by the International Union of Pure and Applied Chemistry.
It is a
rarely used name of water, and mostly used in various hoaxes or spoofs that
call for this "lethal chemical" to be banned, such as in the
dihydrogen monoxide hoax.
Other
systematic names for water include hydroxic acid, hydroxylic acid, and hydrogen
hydroxide, using acid and base names.
None of these exotic names are used
widely.
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 is a tasteless, odorless liquid at
ambient temperature and pressure, and appears colorless in small quantities,
although it has its own intrinsic very light blue hue.
Ice also
appears colorless, and water vapor is essentially invisible as a gas.
Water is
primarily a liquid under standard conditions, which is not predicted from its
relationship to other analogous hydrides of the oxygen family in the periodic
table, which are gases such as hydrogen sulfide. The elements surrounding
oxygen in the periodic table, nitrogen, fluorine, phosphorus, sulfur and
chlorine, all combine with hydrogen to produce gases under standard conditions.
The reason
that water forms a liquid is that oxygen is more electronegative than all of
these elements with the exception of fluorine.
Oxygen
attracts electrons much more strongly than hydrogen, resulting in a net
positive charge on the hydrogen atoms, and a net negative charge on the oxygen
atom. These atomic charges give each water molecule a net dipole moment. Electrical
attraction between water molecules due to this dipole pulls individual
molecules closer together, making it more difficult to separate the molecules
and therefore raising the boiling point. This attraction is known as hydrogen
bonding.
The molecules
of water are constantly moving in relation to each other, and the hydrogen
bonds are continually breaking and reforming at timescales faster than 200
femtoseconds (2×10−13 seconds).
However, these
bonds are strong enough to create many of the peculiar properties of water,
some of which make it integral to life.
Water can be
described as a polar liquid that slightly dissociates disproportionately or
self ionizes into an hydronium ion and hydroxide ion.
- 2 H
2O ⇌ H
3O+ + OH−
The
dissociation constant for this dissociation is commonly symbolized as Kw and
has a value of about 10−14 at 25 °C; see here for values at other temperatures.
Water, ice, and vapor
Like many
substances, water can take numerous forms, which are broadly categorized by
phase of matter. The liquid phase is the most common among water's phases
(within the Earth's atmosphere and surface) and is the form that is generally
denoted by the word "water". The solid phase of water is known as ice
and commonly takes the structure of hard, amalgamated crystals, such as ice
cubes, or loosely accumulated granular crystals, like snow. For a list of the
many different crystalline and amorphous forms of solid H
2O, see the
article ice. The gaseous phase of water is known as water vapor (or steam), in
which water takes the form of a transparent cloud. (Visible steam and clouds
are, in fact, water in the liquid form as minute droplets suspended in the
air.)
The fourth
state of water that of a supercritical fluid, is much less common than the
other three and only rarely occurs in nature, in extremely hostile conditions.
When water achieves a specific critical temperature and a specific critical
pressure (647 K and 22.064 MPa), the liquid and gas phases merge to one
homogeneous fluid phase, with properties of both gas and liquid. A likely
example of naturally occurring supercritical water is in the hottest parts of
deep water hydrothermal vents, in which water is heated to the critical
temperature by volcanicplumes and the critical pressure is caused by the weight
of the ocean at the extreme depths where the vents are located. This pressure
is reached at a depth of about 2200 meters: much less than the mean depth of the
ocean (3800 meters).
Heat capacity and heats of vaporization and fusion
Water has a
very high specific heat capacity of 4.1814 J/(g·K) at 25 °C – the second
highest among all the heteroatomic species (after ammonia), as well as a high
heat of vaporization (40.65 kJ/mol or 2257 kJ/kg at the normal boiling point),
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. According to Josh Willis, of
NASA's Jet Propulsion Laboratory, the oceans can absorb one thousand times more
heat than the atmosphere without changing their temperature much and are
absorbing 80 to 90% of the heat global warming.
The specific
enthalpy of fusion (more commonly known as latent heat) of water is 333.55
kJ/kg at 0 °C: the same amount of energy is required to melt ice as to warm ice
from −160 °C up to its melting point or to heat the same amount of water by
about 80 °C. Of common substances, only that of ammonia is higher. This
property confers resistance to melting on the ice of glaciers and drift ice.
Before and since the advent of mechanical refrigeration, ice was and still is
in common use for retarding food spoilage.
The specific
heat capacity of ice at −10 °C is 2.03 J/(g·K) and the heat capacity of steam
at 100 °C is 2.08 J/(g·K).
Density of water and ice
.svg)
The density of
water is about 1 gram per cubic centimetre (62 lb/cu ft): this relationship was
originally used to define the gram.
The density
varies with temperature, but not linearly: as the temperature increases, the
density rises to a peak at 3.98 °C (39.16 °F) and then decreases.
This unusual
negative thermal expansion below 4 °C (39 °F) is also observed in molten
silica. Regular, hexagonal ice is also less dense than liquid water—upon
freezing, the density of water decreases by about 9%.
These effects
are due to the reduction of thermal motion with cooling, which allows water
molecules to form more hydrogen bonds that prevent the molecules from coming
close to each other.
While below 4 °C the breakage of hydrogen
bonds due to heating allows water molecules to pack closer despite the increase
in the thermal motion (which tends to expand a liquid), above 4 °C water
expands as the temperature increases.
Water near the boiling point is about 4% less
dense than water at 4 °C (39 °F).
Other
substances that expand on freezing are acetic acid, silicon, gallium,
germanium, bismuth, plutonium and also chemical compounds that form spacious
crystal lattices with tetrahedral coordination.
Under
increasing pressure, ice undergoes a number of transitions to other allotropic
forms with higher density than liquid water, such as ice II, ice III,
high-density amorphous ice (HDA), and very-high-density amorphous ice (VHDA).
The unusual
density curve and lower density of ice than of water is vital to life—if water
was most dense at the freezing point, then in winter the very cold water at the
surface of lakes and other water bodies would sink, the lake could freeze from
the bottom up, and all life in them would be killed.
Furthermore,
given that water is a good thermal insulator (due to its heat capacity), some
frozen lakes might not completely thaw in summer.
The layer of
ice that floats on top insulates the water below.
Water at about
4 °C (39 °F) also sinks to the bottom, thus keeping the temperature of the
water at the bottom constant (see diagram).
Density of saltwater and ice
The density of
salt water depends on the dissolved salt content as well as the temperature.
Ice still floats in the oceans, otherwise they would freeze from the bottom up.
However, the salt content of oceans lowers the freezing point by about 1.9 °C
(see here for explanation) and lowers the temperature of the density maximum of
water to the freezing point. This is why, in ocean water, the downward
convection of colder water is not blocked by an expansion of water as it
becomes colder near the freezing point. The oceans' cold water near the
freezing point continues to sink. So creatures that live at the bottom of cold
oceans like the Arctic Ocean generally live in water 4 °C colder than at the
bottom of frozen-over fresh water lakes and rivers.
As the surface
of salt water begins to freeze (at −1.9 °C for normal salinity seawater, 3.5%)
the ice that forms is essentially salt-free, with about the same density as
freshwater ice. This ice floats on the surface, and the salt that is "frozen
out" adds to the salinity and density of the sea water just below it, in a
process known as brine rejection. This denser salt water sinks by convection
and the replacing seawater is subject to the same process. This produces
essentially freshwater ice at −1.9 °C on the surface. The increased density
of the sea water beneath the forming ice causes it to sink towards the bottom.
On a large scale, the process of brine rejection and sinking cold salty water
results in ocean currents forming to transport such water away from the Poles,
leading to a global system of currents called the thermohaline circulation.
Miscibility and condensation
Water is
miscible with many liquids, for example 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.
This can be predicted by comparing the polarity. Water being a relatively polar
compound will tend to be miscible with liquids of high polarity such as ethanol
and acetone, whereas compounds with low polarity will tend to be immiscible and
poorly soluble such as with hydrocarbons.
As a gas,
water vapor is completely miscible with air. On the other hand, the maximum
water vapor pressure that is thermodynamically stable with the liquid (or
solid) at a given temperature is relatively low compared with total atmospheric
pressure. For example, if the vapor's partial pressure is 2% of atmospheric
pressure and the air is cooled from 25 °C, starting at about 22 °C water will
start to condense, defining the dew point, and creating fog or dew. The reverse
process accounts for the fog burning off in the morning. If the humidity is
increased at room temperature, for example, by running a hot shower or a bath,
and the temperature stays about the same, the vapor soon reaches the pressure
for phase change, and then condenses out as minute water droplets, commonly
referred to as steam.
A gas in this
context is referred to as saturated or 100% relative humidity, when the vapor
pressure of water in the air is at the equilibrium with vapor pressure due to
(liquid) water; water (or ice, if cool enough) will fail to lose mass through
evaporation when exposed to saturated air. Because the amount of water vapor in
air is small, relative humidity, the ratio of the partial pressure due to the
water vapor to the saturated partial vapor pressure, is much more useful. Water
vapor pressure above 100% relative humidity is called super-saturated and can
occur if air is rapidly cooled, for example, by rising suddenly in an updraft.
Vapor pressure
The compressibility of water is a function of pressure and temperature. At 0 °C, at the limit of zero pressure, the compressibility is 5.1×10−10 Pa−1. At the zero-pressure limit, the compressibility reaches a minimum of 4.4×10−10 Pa−1 around 45 °C before increasing again with increasing temperature. As the pressure is increased, the compressibility decreases, being 3.9×10−10 Pa−1 at 0 °C and 100 megapascals (1,000 bar).Compressibility
The bulk modulus of water is about 2.2 GPa.
The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[41]
Triple point
| Phases in stable equilibrium | Pressure | Temperature |
|---|---|---|
| liquid water, ice Ih, and water vapor | 611.657 Pa | 273.16 K (0.01 °C) |
| liquid water, ice Ih, and ice III | 209.9 MPa | 251 K (−22 °C) |
| liquid water, ice III, and ice V | 350.1 MPa | −17.0 °C |
| liquid water, ice V, and ice VI | 632.4 MPa | 0.16 °C |
| ice Ih, Ice II, and ice III | 213 MPa | −35 °C |
| ice II, ice III, and ice V | 344 MPa | −24 °C |
| ice II, ice V, and ice VI | 626 MPa | −70 °C |
The temperature and pressure at which solid, liquid, and gaseous water coexist in equilibrium is called the triple point of water. This point is used to define the units of temperature (the kelvin, the SI unit of thermodynamic temperature and, indirectly, the degree Celsius and even the degree Fahrenheit).
As a consequence, water's triple point temperature, as measured in these units, is a prescribed value rather than a measured quantity.
This pressure is quite low, about 1⁄166 of the normal sea level barometric pressure of 101,325 Pa. The atmospheric surface pressure on planet Mars is 610.5 Pa, which is remarkably close to the triple point pressure. The altitude of this surface pressure was used to define zero-elevation or "sea level" on that planet.[43]
Although it is commonly named as "the triple point of water", the stable combination of liquid water, ice I, and water vapor is but one of several triple points on the phase diagram of water. Gustav Heinrich Johann Apollon Tammann in Göttingen produced data on several other triple points in the early 20th century. Kamb and others documented further triple points in the 1960s.
Melting point
The melting point of ice is 0 °C (32 °F; 273 K) at standard pressure; however, pure liquid water can be supercooled well below that temperature without freezing if the liquid is not mechanically disturbed. It can remain in a fluid state down to its homogeneous nucleation point of about 231 K (−42 °C; −44 °F).[47] The melting point of ordinary hexagonal ice falls slightly under moderately high pressures, by 0.0073 °C (0.0131 °F)/atm[f] or about 0.5 °C (0.90 °F)/70 atm as the stabilization energy of hydrogen bonding is exceeded by intermolecular repulsion, but as ice transforms into its allotropes (see crystalline states of ice) above 209.9 MPa (2,072 atm), the melting point increases markedly with pressure, i.e., reaching 355 K (82 °C) at 2.216 GPa (21,870 atm) (triple point of Ice VII
Electrical properties
Electrical conductivity
Pure water containing no exogenous ions is an excellent insulator, but not even "deionized" water is completely free of ions. Water undergoes auto-ionization in the liquid state, when two water molecules form one hydroxide anion (OH− ) and one hydronium cation (H
3O+ ).
3O+ ).
Because water is such a good solvent, it almost always has some solute dissolved in it, often a salt. If water has even a tiny amount of such an impurity, then it can conduct electricity far more readily.
It is known that the theoretical maximum electrical resistivity for water is approximately 18.2 MΩ·cm (182 kΩ·m) at 25 °C.This figure agrees well with what is typically seen on reverse osmosis, ultra-filtered and deionized ultra-pure water systems used, for instance, in semiconductor manufacturing plants. A salt or acid contaminant level exceeding even 100 parts per trillion (ppt) in otherwise ultra-pure water begins to noticeably lower its resistivity by up to several kΩ·m.
In pure water, sensitive equipment can detect a very slight electrical conductivity of 0.05501 ± 0.0001 µS/cm at 25.00 °C. Water can also be electrolyzed into oxygen and hydrogen gases but in the absence of dissolved ions this is a very slow process, as very little current is conducted. In ice, the primary charge carriers are protons (see proton conductor). Ice was previously thought to have a small but measurable conductivity of 1×10−10 S cm−1, but this conductivity is now thought to be almost entirely from surface defects, and without those, ice is an insulator with an immeasurably small conductivity.
Polarity, hydrogen bonding and intermolecular structure
An important feature of water is its polar nature. The structure has a bent molecular geometry for the two hydrogens from the oxygen vertex. The oxygen atom also has two lone pairs of electrons. One effect usually ascribed to the lone pairs is that the H–O–H gas phase bend angle is 104.48°, which is smaller than the typical tetrahedral angle of 109.47°. The lone pairs are closer to the oxygen atom than the electrons sigma bonded to the hydrogens, so they require more space. The increased repulsion of the lone pairs forces the O–H bonds closer to each other.
Another effect of the electronic structure is that water is a polar molecule. Due to the difference in electronegativity, there is a bond dipole moment pointing from each H to the O, making the oxygen partially negative and each hydrogen partially positive. In addition, the lone pairs of electrons on the O are in the direction opposite to the hydrogen atoms. This results in a large molecular dipole, pointing from a positive region between the two hydrogen atoms to the negative region of the oxygen atom. The charge differences cause water molecules to be attracted to each other (the relatively positive areas being attracted to the relatively negative areas) and to other polar molecules. This attraction contributes to hydrogen bonding, and explains many of the properties of water, such as solvent action.
Although hydrogen bonding is a relatively weak attraction compared to the covalent bonds within the water molecule itself, it is responsible for a number of water's physical properties. These properties include its relatively high melting and boiling point temperatures: more energy is required to break the hydrogen bonds between water molecules. In contrast, hydrogen sulfide (H
2S), has much weaker hydrogen bonding due to sulfur's lower electronegativity. H
2S is a gas at room temperature, in spite of hydrogen sulfide having nearly twice the molar mass of water. The extra bonding between water molecules also gives liquid water a large specific heat capacity. This high heat capacity makes water a good heat storage medium (coolant) and heat shield.
2S), has much weaker hydrogen bonding due to sulfur's lower electronegativity. H
2S is a gas at room temperature, in spite of hydrogen sulfide having nearly twice the molar mass of water. The extra bonding between water molecules also gives liquid water a large specific heat capacity. This high heat capacity makes water a good heat storage medium (coolant) and heat shield.
Proposed structures
A single water molecule can participate in a maximum of four hydrogen bonds because it can accept two bonds using the lone pairs on oxygen and donate two hydrogen atoms. Other molecules like hydrogen fluoride, ammonia and methanol can also form hydrogen bonds. However, they do not show anomalous thermodynamic, kinetic or structural properties like those observed in water because none of them can form four hydrogen bonds: either they cannot donate or accept hydrogen atoms, or there are steric effects in bulky residues. In water, intermolecular tetrahedral structures form due to the four hydrogen bonds, thereby forming an open structure and a three-dimensional bonding network, resulting in the anomalous decrease in density when cooled below 4 °C. This repeated, constantly reorganizing unit defines a three-dimensional network extending throughout the liquid. This view is based upon neutron scattering studies and computer simulations, and it makes sense in the light of the unambiguously tetrahedral arrangement of water molecules in ice structures.
However, there is an alternative theory for the structure of water. In 2004, a controversial paper from Stockholm University suggested that water molecules in liquid form typically bind not to four but to only two others; thus forming chains and rings. The term "string theory of water" (which is not to be confused with the string theory of physics) was coined. These observations were based upon X-ray absorption spectroscopy that probed the local environment of individual oxygen atoms. Water, the team suggests, is a muddle of the two proposed structures. They say that it is a soup flecked with "icebergs" each comprising 100 or so loosely connected molecules that are relatively open and hydrogen bonded. The soup is made of the string structure and the icebergs of the tetrahedral structure.[55]
Cohesion and adhesion
Water molecules stay close to each other (cohesion), due to the collective action of hydrogen bonds between water molecules. These hydrogen bonds are constantly breaking, with new bonds being formed with different water molecules; but at any given time in a sample of liquid water, a large portion of the molecules are held together by such bonds.
Water also has high adhesion properties because of its polar nature. On extremely clean/smooth glass the water may form a thin film because the molecular forces between glass and water molecules (adhesive forces) are stronger than the cohesive forces. In biological cells and organelles, water is in contact with membrane and protein surfaces that are hydrophilic; that is, surfaces that have a strong attraction to water. Irving Langmuir observed a strong repulsive force between hydrophilic surfaces. To dehydrate hydrophilic surfaces—to remove the strongly held layers of water of hydration—requires doing substantial work against these forces, called hydration forces. These forces are very large but decrease rapidly over a nanometer or less. They are important in biology, particularly when cells are dehydrated by exposure to dry atmospheres or to extracellular freezing.
Surface tension
Water has a high surface tension of 71.99 mN/m at 25 °C,
caused by the strong cohesion between water molecules, the highest of the common non-ionic, non-metallic liquids. This can be seen when small quantities of water are placed onto a sorption-free (non-adsorbent and non-absorbent) surface, such as polyethylene or Teflon, and the water stays together as drops. Just as significantly, air trapped in surface disturbances forms bubbles, which sometimes last long enough to transfer gas molecules to the water.
Another surface tension effect is capillary waves, which are the surface ripples that form around the impacts of drops on water surfaces, and sometimes occur with strong subsurface currents flowing to the water surface. The apparent elasticity caused by surface tension drives the waves. Additionally, the surface tension of water allows certain insects to walk on the surface of water. This is caused by the strength of the hydrogen bonds, making it difficult to break the surface of water. These insects, including the raft spider, are more dense than water and yet are still able to walk on the surface.
Capillary action
Due to an interplay of the forces of adhesion and surface tension, water exhibits capillary action whereby water rises into a narrow tube against the force of gravity. Water adheres to the inside wall of the tube and surface tension tends to straighten the surface causing a surface rise and more water is pulled up through cohesion. The process continues as the water flows up the tube until there is enough water such that gravity balances the adhesive force.
Surface tension and capillary action are important in biology. For example, when water is carried through xylem up stems in plants, the strong intermolecular attractions (cohesion) hold the water column together and adhesive properties maintain the water attachment to the xylem and prevent tension rupture caused by transpiration pull.
Water as a solvent
Water is also a good solvent, due to its polarity. Substances that will mix well and dissolve in water (e.g. salts) 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 ability of a substance to dissolve in water is determined by whether or not the substance can match or better the strong attractive forces that water molecules generate between other water molecules. If a substance has properties that do not allow it to overcome these strong intermolecular forces, the molecules are "pushed out" from the water, and do not dissolve. Contrary to the common misconception, water and hydrophobic substances do not "repel", and the hydration of a hydrophobic surface is energetically, but not entropically, favorable.
When an ionic or polar compound enters water, it is surrounded by water molecules (hydration). The relatively small size of water molecules (~ 3 angstroms) allows many water molecules to surround one molecule of solute. The partially negative dipole ends of the water are attracted to positively charged components of the solute, and vice versa for the positive dipole ends.
In general, ionic and polar substances such as acids, alcohols, and salts are relatively soluble in water, and non-polar substances such as fats and oils are not. Non-polar molecules stay together in water because it is energetically more favorable for the water molecules to hydrogen bond to each other than to engage in van der Waals interactions with non-polar molecules.
An example of an ionic solute is table salt; the sodium chloride, NaCl, separates into Na+ cations and Cl− anions, each being surrounded by water molecules. The ions are then easily transported away from their crystalline lattice into solution. An example of a nonionic solute is table sugar. The water dipoles make hydrogen bonds with the polar regions of the sugar molecule (OH groups) and allow it to be carried away into solution.
Quantum tunneling
Main article: Quantum tunneling of water
The quantum tunneling dynamics in water was reported as early as 1992. At that time it was known that there are motions which destroy and regenerate the weak hydrogen bond by internal rotations of the substituent water monomers.[61] On 18 March 2016, it was reported that the hydrogen bond can be broken by quantum tunneling in the water hexamer. Unlike previously reported tunneling motions in water, this involved the concerted breaking of two hydrogen bonds.[62] Later in the same year, the discovery of the quantum tunneling of water molecules was reported.[63]
Chemical properties in nature
Geochemistry
Action of water on rock over long periods of time typically leads to weathering and water erosion, physical processes that convert solid rocks and minerals into soil and sediment, but under some conditions chemical reactions with water occur as well, resulting in metasomatism or mineral hydration, a type of chemical alteration of a rock which produces clay minerals. It also occurs when Portland cement hardens.
Water ice can form clathrate compounds, known as clathrate hydrates, with a variety of small molecules that can be embedded in its spacious crystal lattice. The most notable of these is methane clathrate, 4 CH
4·23H
2O, naturally found in large quantities on the ocean floor.
4·23H
2O, naturally found in large quantities on the ocean floor.
Acidity
Pure water has the concentration of hydroxide ions (OH−) equal to that of the hydronium (H
3O+) or hydrogen (H+) ions, which gives pH of 7 at 298 K. In practice, pure water is very difficult to produce. Water left exposed to air for any length of time will dissolve carbon dioxide, forming a dilute solution of carbonic acid, with a limiting pH of about 5.7. As cloud droplets form in the atmosphere and as raindrops fall through the air minor amounts of CO
2 are absorbed, and thus most rain is slightly acidic. If high amounts of nitrogen and sulfur oxides are present in the air, they too will dissolve into the cloud and rain drops, producing acid rain.
3O+) or hydrogen (H+) ions, which gives pH of 7 at 298 K. In practice, pure water is very difficult to produce. Water left exposed to air for any length of time will dissolve carbon dioxide, forming a dilute solution of carbonic acid, with a limiting pH of about 5.7. As cloud droplets form in the atmosphere and as raindrops fall through the air minor amounts of CO
2 are absorbed, and thus most rain is slightly acidic. If high amounts of nitrogen and sulfur oxides are present in the air, they too will dissolve into the cloud and rain drops, producing acid rain.
Electromagnetic absorption
Main article: Electromagnetic absorption by water
Water is relatively transparent to visible light, near ultraviolet light, and far-red light, but it absorbs most ultraviolet light, infrared light, and microwaves. Most photoreceptors and photosynthetic pigments utilize the portion of the light spectrum that is transmitted well through water. Microwave ovens take advantage of water's opacity to microwave radiation to heat the water inside of foods. The very weak onset of absorption in the red end of the visible spectrum lends water its intrinsic blue hue (see Color of water).
Heavy water and isotopologues
Several isotopes of both hydrogen and oxygen exist, giving rise to several known isotopologues of water.
Hydrogen occurs naturally in three isotopes. The most common isotope, 1H, sometimes called protium, accounts for more than 99.98% of hydrogen in water and consists of only a single proton in its nucleus. A second stable isotope, deuterium (chemical symbol D or 2H), has an additional neutron. Deuterium oxide, D
2O, is also known as heavy water because of its higher density. It is used in nuclear reactors as a neutron moderator. The third isotope, tritium (chemical symbol T or 3H) has 1 proton and 2 neutrons, and is radioactive, decaying with a half-life of 4500 days. THO exists in nature only in minute quantities, being produced primarily via cosmic ray-induced nuclear reactions in the atmosphere. Water with one protium and one deuterium atom HDO occurs naturally in ordinary water in low concentrations (~0.03%) and D
2O in far lower amounts (0.000003%) and any such molecules are temporary as the atoms recombine.
2O, is also known as heavy water because of its higher density. It is used in nuclear reactors as a neutron moderator. The third isotope, tritium (chemical symbol T or 3H) has 1 proton and 2 neutrons, and is radioactive, decaying with a half-life of 4500 days. THO exists in nature only in minute quantities, being produced primarily via cosmic ray-induced nuclear reactions in the atmosphere. Water with one protium and one deuterium atom HDO occurs naturally in ordinary water in low concentrations (~0.03%) and D
2O in far lower amounts (0.000003%) and any such molecules are temporary as the atoms recombine.
The most notable physical differences between H
2O and D
2O, other than the simple difference in specific mass, involve properties that are affected by hydrogen bonding, such as freezing and boiling, and other kinetic effects. This is because the nucleus of deuterium is twice as heavy as protium, and this causes noticeable differences in bonding energies. The difference in boiling points allows the isotopologues to be separated. The self-diffusion coefficient of H
2O at 25 °C is 23% higher than the value of D
2O.[64] Because water molecules exchange hydrogen atoms with one another, hydrogen deuterium oxide (DOH) is much more common in low-purity heavy water than pure dideuterium monoxide D
2O.
2O and D
2O, other than the simple difference in specific mass, involve properties that are affected by hydrogen bonding, such as freezing and boiling, and other kinetic effects. This is because the nucleus of deuterium is twice as heavy as protium, and this causes noticeable differences in bonding energies. The difference in boiling points allows the isotopologues to be separated. The self-diffusion coefficient of H
2O at 25 °C is 23% higher than the value of D
2O.[64] Because water molecules exchange hydrogen atoms with one another, hydrogen deuterium oxide (DOH) is much more common in low-purity heavy water than pure dideuterium monoxide D
2O.
Consumption of pure isolated D
2O may affect biochemical processes – ingestion of large amounts impairs kidney and central nervous system function. Small quantities can be consumed without any ill-effects; humans are generally unaware of taste differences,[65] but sometimes report a burning sensation[66] or sweet flavor.[67] Very large amounts of heavy water must be consumed for any toxicity to become apparent. Rats, however, are able to avoid heavy water by smell, and it is toxic to many animals.[68]
2O may affect biochemical processes – ingestion of large amounts impairs kidney and central nervous system function. Small quantities can be consumed without any ill-effects; humans are generally unaware of taste differences,[65] but sometimes report a burning sensation[66] or sweet flavor.[67] Very large amounts of heavy water must be consumed for any toxicity to become apparent. Rats, however, are able to avoid heavy water by smell, and it is toxic to many animals.[68]
Oxygen also has three stable isotopes, with 16O present in 99.76%, 17O in 0.04%, and 18O in 0.2% of water molecules.[69]
Light water refers to deuterium-depleted water (DDW), water in which the deuterium content has been reduced below the standard 155 ppm level.
Standard water
Vienna Standard Mean Ocean Water is the current international standard for water isotopes. Naturally occurring water is almost completely composed of the neutron-less hydrogen isotope protium. Only 155 ppm include deuterium (2H or D), a hydrogen isotope with one neutron, and fewer than 20 parts per quintillion include tritium (3H or T), which has two neutrons.
Reactions
Acid-base reactions
Water is amphoteric: it has the ability to act as either an acid or a base in chemical reactions.[70] According to the Brønsted-Lowry definition, an acid is a proton (H+) donor and a base is a proton acceptor.[71] When reacting with a stronger acid, water acts as a base; when reacting with a stronger base, it acts as an acid.[71] For instance, water receives an H+ ion from HCl when hydrochloric acid is formed:
- HCl
(acid) + H
2O
(base) ⇌ H
3O+ + Cl−
In the reaction with ammonia, NH
3, water donates a H+ ion, and is thus acting as an acid:
3, water donates a H+ ion, and is thus acting as an acid:
- NH
3
(base) + H
2O
(acid) ⇌ NH+
4 + OH−
Because the oxygen atom in water has two lone pairs, water often acts as a Lewis base, or electron pair donor, in reactions with Lewis acids, although it can also react with Lewis bases, forming hydrogen bonds between the electron pair donors and the hydrogen atoms of water. HSAB theory describes water as both a weak hard acid and a weak hard base, meaning that it reacts preferentially with other hard species:
- H+
(Lewis acid) + H
2O
(Lewis base) → H
3O+
- Fe3+
(Lewis acid) + H
2O
(Lewis base) → Fe(H
2O)3+
6
- Cl−
(Lewis base) + H
2O
(Lewis acid) → Cl(H
2O)−
6
When a salt of a weak acid or of a weak base is dissolved in water, water can partially hydrolyze the salt, producing the corresponding base or acid, which gives aqueous solutions of soap and baking soda their basic pH:
- Na
2CO
3 + H
2O ⇌ NaOH + NaHCO
3
Ligand chemistry
Water's Lewis base character makes it a common ligand in transition metal complexes, examples of which range from solvated ions, such as Fe(H
2O)3+
6, to perrhenic acid, which contains two water molecules coordinated to a rhenium atom, and various solid hydrates, such as CoCl
2·6H
2O. Water is typically a monodentate ligand–it forms only one bond with the central atom.[72]
2O)3+
6, to perrhenic acid, which contains two water molecules coordinated to a rhenium atom, and various solid hydrates, such as CoCl
2·6H
2O. Water is typically a monodentate ligand–it forms only one bond with the central atom.[72]
Organic chemistry
As a hard base, water reacts readily with organic carbocations; for example in an hydration reaction, a hydroxyl group (OH−) and an acidic proton are added to the two carbon atoms bonded together in the carbon-carbon double bond, resulting in an alcohol. When addition of water to an organic molecule cleaves the molecule in two, hydrolysis is said to occur. Notable examples of hydrolysis are the saponification of fats and the digestion of proteins and polysaccharides. Water can also be a leaving group in SN2 substitution and E2 elimination reactions; the latter is then known as a dehydration reaction.
Water in redox reactions
Water contains hydrogen in the oxidation state +1 and oxygen in the oxidation state −2.[73] It oxidizes chemicals such as hydrides, alkali metals, and some alkaline earth metals.[74][75][76] One example of an alkali metal reacting with water is:[77]
- 2 Na + 2 H
2O → H
2 + 2 Na+ + 2 OH−
Some other reactive metals, such as aluminum and beryllium, are oxidized by water as well, but their oxides adhere to the metal and form a passive protective layer.[78] Note, however, that the rusting of iron is a reaction between iron and oxygen[79] that is dissolved in water, not between iron and water.
Water can be oxidized to emit oxygen gas, but very few oxidants react with water even if their reduction potential is greater than the potential of O
2/H
2O. Almost all such reactions require a catalyst.[80] An example of the oxidation of water is:
2/H
2O. Almost all such reactions require a catalyst.[80] An example of the oxidation of water is:
- 4 AgF
2 + 2 H
2O → 4 AgF + 4 HF + O
2
Electrolysis
Water can be split into its constituent elements, hydrogen and oxygen, by passing an electric current through it. This process is called electrolysis. The cathode half reaction is:
- 2 H+ + 2
e− → H
2
The anode half reaction is:
- 2 H
2O → O
2 + 4 H+ + 4
e−
The gases produced bubble to the surface, where they can be collected. The standard potential of the water electrolysis cell (when heat is added to the reaction) is a minimum of 1.23 V at 25 °C. The operating potential is actually 1.48 V (or above) in practical electrolysis when heat input is negligible.
Source: en.wikipedia.org
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