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Water
Article by: Harold L. Friedman. Department of Chemistry, State University of New York, Stony Brook, New York.
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A clear, odorless, tasteless liquid at 0 to 100°C, formula H2O, also known as
dihydrogen oxide. Water also exists in the vapor (gaseous) and solid (ice) states under
normal terrestrial conditions and is a solvent for many substances (Fig. 1). Water is
essential for life and is the most abundant molecule on Earth. Water vapor is a green-
house gas. On Earth, water vapor and carbon dioxide provide most of the greenhouse
warming to maintain a global mean surface temperature of about 15°C, allowing for
liquid water.
Formation
It is formed by the direct reaction (1) of hydrogen with oxygen.
2H2 + O2 —> 2H2O
The other compound of hydrogen and oxygen, hydrogen peroxide, readily decom-
poses to form water, reaction (2).
2H2 + O2 —> 2H2O + O2
Water also is formed in the combustion of hydrogen-containing compounds, in the
pyrolysis of hydrates, and in animal metabolism. Some properties of water are given in
the table.
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Content
• Formation
• Gaseous state
• Solid state
• Liquid state
• Solutions in water
• Chemical properties
Key Concepts
• Water (chemical formula H2O) is the most abundant molecule on Earth.
• Water can occur as a solid (ice), a liquid, or a gas (water vapor), depending
on environmental temperature and pressure.
• The net attraction that holds water molecules together is called hydrogen bonding.
• The unusual strength of hydrogen bonds results in water's many unique properties,
such as ice's low density and buoyancy, and liquid water's extremely high boiling
point, high heat of vaporization, and high electrical conductivity.
• Water is an excellent solvent for many substances but is not a strong oxidizing or
reducing agent.
• Strong acids and bases react with water.
Fig. 1: Molecular model of water. Hydrogen is shown in
white and oxygen is shown in red.
