Water

Properties

General
Name	Water
Chemical formula	H2O
Appearance	Colourless liquid
Physical
Formula weight	18.01528 amu
Melting point	273 K (0 °C)
Boiling point	373 K (100 °C)
Density	1.0 ×103 kg/m3
Thermochemistry
ΔfH0gas	-241.83 kJ/mol
ΔfH0liquid	-285.83 kJ/mol
ΔfH0solid	-291.83 kJ/mol
S0gas, 1 bar	188.84 J/mol·K
S0liquid, 1 bar	69.95 J/mol·K
S0solid	41 J/mol·K
Safety
Ingestion	Not dangerous.
Inhalation	not dangerous
Skin	Prolonged immersion may cause flaking (desquamation).
Eyes	Not dangerous.
More info	None.
SI units were used where possible. Unless otherwise stated, standard conditions were used. Disclaimer and references

Water (dihydrogen monoxide, hydrogen hydroxide, hydrogen oxide, hydronium hydroxide, hydroxilic acid, or hydroxic acid) is a chemical compound which is liquid at room temperature and standard pressure. It has the chemical formula H₂O, meaning that one molecule of water is composed of 2 hydrogen atoms and one oxygen atom. Water is found almost everywhere on earth and is required by all known life. About 70% of the earth's surface is covered by water.

The solid state of water is known as (water) ice; the gaseous state is known as steam. The units of temperature (formerly the degree Celsius and now the Kelvin) are defined in terms of the triple point of water, 273.16 K (0.01 °C) and 611.2 Pa, the temperature and pressure at which solid, liquid, and gaseous water coexist in equilibrium.

Bodies of water, generic: sea, lake, ocean, river, canal, pond, etc. See water (resource) for information about fresh water supplies.

Chemists sometimes jokingly refer to water as dihydrogen monoxide or DHMO (see http://www.dhmo.org/ (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.

Table of contents 1 The dipolar nature of water 2 Water as a solvent 3 Cohesion and surface tension 4 Conductivity 5 Electrolysis 6 Reactivity 7 Purifying water 8 Mythology 9 Water development 10 See also 11 External links

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 °C. This has the effect that the water at the bottom of lakes in winter typically has a temperature of 4 °C, 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 crystaline latice 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 have too much salt or other impurities in it. Common impurities include toxic chemicals or harmful bacteria. Some solutes are acceptable and even desirable for perceived taste enhancement.

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 development

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.

See also

• Dehydration
• Drinking water
• Drought
• Flood
• Heavy water
• Hydrography
• Hydrology
• Polywater theory
• Precipitation
• Trasvasement

External links

• World Water Forum (http://www.worldwaterforum.org)
• World Water Assessment Program (http://www.unesco.org/water/wwap/)
• United Nations' World Water Development Report (http://unesdoc.unesco.org/images/0012/001295/129556e.pdf)
• Can hot water freeze faster than cold water?...I. Yes -- a general explanation (http://www.physics.adelaide.edu.au/~dkoks/Faq/General/hot_water) Citat: "...This phenomenon is extremely counter- intuitive, and surprising even to most scientists, but it is in fact real.....called the Mpemba effect...It is still not known exactly why this happens...."