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Water (molecule)

physical and chemical properties of pure water
Water (H2O)
The water molecule has this basic geometric structureWater molecules have this structure.
Systematic name Water
Other names Aqua
Hydrogen oxide
Hydrogen hydroxide
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[1]
CAS number [7732-18-5]
see also Water (data page)
Density and phase 1000 kg·m−3, liquid (4 °C)
917 kg·m−3, solid
Melting point °C, 32 °F (273.15 K)[2]
Boiling point 100 °C, 212 °F (373.15 K)[2]
Triple point 273.16 K, 611.73 Pa
Critical point 647 K, 22.1 MPa
Specific heat
cp=1970 J·kg−1·K−1 @ 300 K
cv=1510 J·kg−1·K−1 @ 300 K[3]
Specific heat
4186 J·kg−1·K−1
Specific heat
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
Molecular shape non-linear bent
Crystal structure Hexagonal
See ice
Dipole moment 1.85 D
MSDS External MSDS
Main hazards Drowning
NFPA 704

NFPA 704.svg

RTECS number ZC0110000
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Related solvents acetone
Related compounds water vapor
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.[4]

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 waterEdit

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.


  1. Braun C.L & Smitnov S.N. 1993. Why is water blue? J. Chem Ed. 70, 612. [1]
  2. 2.0 2.1 Vienna Standard Mean Ocean Water (VSMOW), used for calibration, melts at 273.1500089(10) K (0.000089(10) °C, and boils at 373.1339 K (99.9839 °C)
  3. Serway, Raymond A. Physics for scientists and engineers, 3rd edition
  4. Water chemistry [2]

Other websitesEdit