Water

Water is a chemical compound which is
liquid at room temperature and
standard pressure. It has the chemical               General
formula H2O, meaning that one molecule Name             Water
of water is composed of 2 hydrogen     Chemical formula H2O
atoms and one oxygen atom. Water is
found almost everywhere on earth and   Appearance       Colourless liquid
is required by all known life. About                 Physical
70% of the earth's surface is covered  Formula weight   18.01528 amu
by water. Sometimes water may fall
from the sky of Planet Earth in the    Melting point    273 K (0 ˇC)
form of rain.                          Boiling point    373 K (100 ˇC)
                                       Critical
                                       temperature      674 K
                                       Critical
                                       Pressure         22.1x10^6?? Pa
                                                        1.0 ×103
                                                 Thermochemistry
                                       ΔfH0gas    -241.83 kJ/mol
                                       ΔfH0liquid -285.83 kJ/mol
                                       ΔfH0solid  -291.83 kJ/mol
                                       S0gas, 1 bar     188.84 J/moláK

General                                S0liquid, 1 bar  69.95 J/moláK

The solid state of water is known as   S0solid          41 J/moláK
(water) ice; the gaseous state is
known as steam. The units of                          Safety
temperature (formerly the degree
Celsius and now the Kelvin) are                         Necessary to life;
defined in terms of the triple point                    excessive
of water, 273.16 K (0.01 ˇC) and 611.2                  consumption can
Pa, the temperature and pressure at    Ingestion        cause headache,
which solid, liquid, and gaseous water                  confusion, and
coexist in equilibrium.                                 cramps, and can be
                                                        fatal in athletes
At temperatures greater than 647 K and                  Non-toxic. Can
pressures greater than 22.064 MPa, a                    dissolve surfactant
collection of water molecules assumes  Inhalation       of lungs.
a supercritical condition, in which                     Suffocation in
liquid-like clusters float within a                     water is called
vapor-like phase.                                       drowning.
                                                        Prolonged immersion
A body of water is a term for an       Skin             may cause flaking
ocean, sea, lake, river, stream,                        (desquamation).
canal, pond, or the like. See water
(resource) for information about fresh Eyes             Not dangerous.
water supplies. See also beach, ferry,  SI units were used where possible.
pier.                                    Unless otherwise stated, standard
                                              conditions were used.
Chemists sometimes jokingly refer to
water as dihydrogen monoxide or DHMO        Disclaimer and references
(see http://www.dhmo.org/), the
systematic covalent name of this molecule, especially in parodies of
chemical research that call for this "lethal chemical" to be banned. The
systematic acid name of water is hydroxic acid or hydroxilic acid, although
these terms are rarely used.

The dipolar nature of water

An important feature of water is its polar nature. The water molecule forms
an angle, with hydrogen atoms at the tips and oxygen at the vertex. Since
oxygen has a higher electronegativity than hydrogen, the side of the
molecule with the oxygen atom has a partial negative charge, relative to the
hydrogen side. A molecule with such a charge difference is called a dipole.
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 is known as hydrogen bonding.

This relatively weak (relative to the covalent bonds within the water
molecule itself) attraction results in properties such as a very high
boiling point, because a lot of heat energy is necessary to break the
hydrogen bonds between molecules, and also a large specific heat capacity.

Also due to hydrogen bonding, water molecules have the peculiar property
that their density in the liquid state is higher than in the crystalline
(solid) state. The highest density of water occurs in the liquid form at a
temperature of 4 ˇC. This has the effect that the water at the bottom of
lakes in winter typically has a temperature of 4 ˇC, allowing fish to
survive. Another consequence is that ice will melt if sufficient pressure is applied.

Water as a solvent

Water is also a good solvent due to its polarity. When an ionic or polar
compound enters water, it is surrounded by water molecules. The relatively
small size of water molecules typically allows many water molecules to
surround one molecule of solute. The partially negative dipoles of the water
are attracted to positively charged components of the solute, and vice versa
for the positive dipoles.

In general, ionic and polar substances such as acids, alcohols, and salts
are easily soluble in water, and nonpolar substances such as fats and oils
are not. Nonpolar molecules stay together in water because it is
energetically more favorable for the the water molecules to hydrogen bond to
each other than to engage in van der Waals interactions with nonpolar 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 hydrogen bond to the dipolar regions of the sugar molecule and
allow it to be carried away into solution.

The solvent properties of water are vital in biology, because many
biochemical reactions take place only in solution (e.g., reactions in the
cytoplasm and blood).

Cohesion and surface tension

The strong hydrogen bonds give water a high cohesiveness and, consequently,
surface tension. This is evident when small quantities of water are put onto
a nonsoluble surface and the water stays together as drops. This feature is
important when water is carried through xylem up stems in plants; the strong
intermolecular attractions hold the water column together, and prevent
tension caused by transpiration pull. Other liquids with lower surface
tension would have a higher tendency to "rip", forming vacuum or air pockets
and rendering the xylem vessel inoperative.

Conductivity

Pure water is actually an insulator, meaning that it does not conduct
electricity well. Because water is such a good solvent, it often has some
solute dissolved in it, most frequently salt. If water has such impurities,
then it can conduct electricity well.

Electrolysis

Water can be split into its constituent elements, hydrogen and oxygen, by
passing a current through it. This process is called electrolysis. Water
molecules naturally disassociate into H+ and OH- ions, which are pulled
toward the cathode and anode, respectively. At the cathode, two H+ ions pick
up electrons and form H2 gas. At the anode, four OH- ions combine and
release O2 gas, molecular water, and four electrons. The gases produced
bubble to the surface, where they can be collected.

Reactivity

Chemically, water is amphoteric: able to act as an acid or base.
Occassionally the term hydroxic acid is used when water acts as an acid in a
chemical reaction. At a pH of 7 (neutral), the concentration of hydroxide
ions (OH-) is equal to that of the hydronium (H3O+) or hydrogen ions (H+)
ions. If the equilibrium is disturbed, the solution becomes acidic (higher
concentration of hydronium ions) or basic (higher concentration of hydroxide ions).

Purifying water

Purified water is needed for many industrial applications, as well as for
consumption. Humans require water that does not contain too much salt or
other impurities. Common impurities include chemicals or harmful bacteria.
Some solutes are acceptable and even desirable for perceived taste
enhancement. Water that is suitable for drinking is termed potable water.

Six popular methods for purifying water are:

  1. Filtering: Water is passed through a sieve that catches small
     particles. The tighter the mesh of the sieve, the smaller the particles
     must be to pass through. Filtering is not sufficient to completely
     purify water, but it is often a necessary first step, since such
     particles can interfere with the more thorough purification methods.
  2. Boiling: Water is heated to its boiling point long enought to
     inactivate or kill microorganisms that normally live in water at room
     temperature. Boiling does not remove solutes that have a lower boiling
     point than the solution, and in fact increases their concentration.
  3. Carbon filtering: Charcoal, a compound that contains a high
     concentration of carbon, absorbs many compounds, including toxic
     compounds. Water is passed through activated charcoal to remove such
     contaminants. This method is most commonly used in household water
     filters and fish tanks.
  4. Distilling: Distillation involves boiling the water to produce water
     vapor. The water vapor then rises to a cooled surface where it can
     condense back into a liquid and be collected. Because the solutes are
     not normally vaporized, they remain in the boiling solution. Even
     distillation does not completely purify water, because of contaminants
     with similar boiling points and droplets of unvaporized liquid carried
     with the steam. Still, 99.9% pure water can be obtained by
     distillation.
  5. Reverse osmosis: Mechanical pressure is applied to an impure solution
     to force pure water through a semi-permeable membrane. The term is
     reverse osmosis, because normal osmosis would result in pure water
     moving in the other direction to dilute the impurities. Reverse osmosis
     is theoretically the most thorough method of large-scale water
     purification available, although perfect semi-permable membranes are
     difficult to create.
  6. Ion exchange chromatography: In this case, water is passed through a
     charged resin column that has side chains that trap calcium, magnesium,
     and other heavy metal ions. In many laboratories, this method of
     purification has replaced distillation, as it provides more quickly a
     high volume of very pure water. Water purified in this way is called
     deionized water.

Mythology

Water is one of the four classical elements along with fire, earth and air,
and was regarded as the ylem, or basic stuff of the universe. Water was
considered cold and moist. In the theory of the four bodily humours, water
was asssociated with phlegm.

Water was also one of the Chinese five elements along with air, fire, wood,
and metal.

Water rights and development

In the United States water law is divided between two legal doctrines:
riparian water rights, used in the eastern and southern states where there
is an abundance of water and the appropriation doctrine (or Colorado
doctrine) used in the arid western states.

UNESCO's World Water Development Report (WWDR, 2003) from its World Water
Assessment Program indicates that in the next 20 years the world is facing
an unprecedented lack of drinking water. The quantity of water available to
everyone is predicted to decrease by 30%. The causes are contamination,
global warming and political problems.

40% of the world's inhabitants have insufficient fresh water for minimal
hygiene. More than 2.2 million people died in 2000 from illnesses related to
the consumption of contaminated water.

The report indicates large global disparities in the raw volume of available
water: from 10 m³ per person per year in Kuwait to 812.121 [m³?]
in French Guiana. However, richer countries such as Kuwait can more easily
cope with low water availability.