ทรัพยากรน้ำ (Water)

Water is a chemical substance with the chemical formula H₂O. A water mol molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state (water vapor or steam). Water also exists in a liquid crystal state near hydrophilic surfaces. Under nomenclature used to name chemical compounds, Dihydrogen monoxide is the scientific name for water, though it is almost never used.

Water covers 70.9% of the Earth's surface, and is vital for all known forms of life. On Earth, 96.5% of the planet's water is found in oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation. Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is contained within biological bodies and manufactured products.

Water (H2O)
IUPAC name Water Oxidane
Other names Hydrogen oxide Dihydrogen monoxide Hydrogen monoxide Dihydrogen oxide Hydrogen hydroxide
Identifiers
CAS number	7732-18-5
RTECS number	ZC0110000
Properties
Molecular formula	H2O
Molar mass	18.01528(33) g/mol
Appearance	white solid or almost colorless, transparent, with a slight hint of blue, crystalline solid or liquid
Density	1000 kg/m3, liquid (4 °C) (62.4 lb/cu. ft) 917 kg/m3, solid
Melting point	0 °C, 32 °F, (273.15 K)
Boiling point	99.98 °C, 211.97 °F (373.13 K)
Acidity (pKa)	15.74 ~35–36
Basicity (pKb)	15.74
Refractive index (nD)	1.3330
Viscosity	0.001 Pa s at 20 °C
Structure
Crystal structure	Hexagonal
Molecular shape	Bent
Hazards
NFPA 704