Water (molecule)

Water (H2O)
General
Systematic name	Water
Other names	Aqua Hydrogen oxide Hydrogen hydroxide Hydrate Oxidane Hydric acid Dihydrogen monoxide Hydrohydroxic acid μ-Oxido dihydrogen
Molecular formula	HOH or H2O
Molar mass	18.01524 g·mol−1
Appearance	transparent, almost colorless liquid with a slight hint of blue
CAS number	[7732-18-5]
see also	Water (data page)
Properties
Density and phase	1000 kg·m−3, liquid (4 °C) 917 kg·m−3, solid
Melting point	0 °C, 32 °F (273.15 K)
Boiling point	100 °C, 212 °F (373.15 K)
Triple point	273.16 K, 611.73 Pa
Critical point	647 K, 22.1 MPa
Specific heat capacity (gas)	cp=1970 J·kg−1·K−1 @ 300 K cv=1510 J·kg−1·K−1 @ 300 K
Specific heat capacity (liquid)	4186 J·kg−1·K−1
Specific heat capacity (solid)	2060 J·kg−1·K−1
Acidity (pKa)	15.74
Basicity (pKb)	15.74
Viscosity	0.001 Pa·s at 20 °C
Surface Tension at 20 °C	7.28 N·m−1
Structure
Molecular shape	non-linear bent
Crystal structure	Hexagonal See ice
Dipole moment	1.85 D
Hazards
MSDS	External MSDS
Main hazards	Drowning
NFPA 704	0 0 0
RTECS number	ZC0110000
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related solvents	acetone methanol
Related compounds	water vapor ice heavy water
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Water (H₂O, HOH) covers 70-75% of the Earth's surface in its liquid and solid (ice) states, and is present in the atmosphere as a vapor. It is the most abundant molecule on the Earth's surface.

At room temperature, it is a nearly colorless, flavorless, and odorless liquid. Many substances dissolve in water and it is commonly seen as the universal solvent; because of this, water in nature and in use is rarely clean, and may have some properties different than those in the laboratory. But there are many compounds that are essentially, if not completely, insoluble in water. Water is the only common, pure substance found naturally in all three states of matter—for other substances, see Chemical properties.

Pure water is tasteless. It is the other chemicals in the water which may give water a flavour.

Anomalous properties of water

Generally, the volume of a liquid increases when heated, however the volume of water decreases when heated within the temperature range of 0°C to 4°C. Its volume increases only when heated above 4°C. Such behavior is one of a number of water's anomalous properties.

This property of volume decrease allows fish and other aquatic animals to survive in a body of water when ice has frozen on the surface. In cold climates, when the temperature of a body of water reaches at 4°C, the layers of water near the top in contact with cold air continue to lose heat energy and their temperature falls below 4°C. On cooling below 4°C, these layers rise rather than sink, as water has maximum density at 4°C. Thus the layer of water with a temperature of 4°C remains at the bottom, while layers of water 3°C, 2°C, 1°C and 0°C are formed above. Because ice is poor conductor of heat, it does not allow heat energy transfer from the liquid water beneath the layers of ice, which prevents the liquid water from freezing. Hence, aquatic creatures survive in such places.

Other websites

• Water Structure and Behaviour A comprehensive and up-to-date NPOV resource maintained by Prof Martin Chaplin of South Bank University, UK
• A spoof site on the "dangers" of dihydrogen monoxide
• Stockholm International Water Institute (SIWI)
• Explanation of the anomalous properties of water
• Computational Chemistry Wiki Archived 2007-09-27 at the Wayback Machine