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Introduction And Basic Water Chemistry
Water (H
2
O, HOH) is the most abundant molecule on Earth's surface,
composing of about 70% of the Earth's surface as liquid and solid state in
addition to being found in the atmosphere as a vapor. It is in dynamic equilibrium
between the liquid and vapor states at standard temperature and pressure. At
room temperature, it is nearly colorless with a hint of blue, tasteless, and
odorless liquid. Many substances dissolve in water and it is commonly referred to
as
the universal solvent . Because of this, water in nature and in use is rarely
pure, and may have some properties different from those in the laboratory.
However, there are many compounds that are essentially, if not completely,
insoluble in water. Water is the only common substance found naturally in all
three common states of matter—for other substances, see Chemical properties.
Water also makes up 75 % of the human body.
Water is the chemical substance with chemical formula H
2
O: one molecule
of water has two hydrogen atoms covalently bonded to a single oxygen atom.
Water is a tasteless, odorless liquid at ambient temperature and pressure, and
appears colorless in small quantities, although it has its own intrinsic very light
blue hue. Ice also appears colorless, and water vapor is essentially invisible as a
gas.
[3]
Water is primarily a liquid under standard conditions, which is not
predicted from its relationship to other analogous hydrides of the oxygen family in
the periodic table, which are gases such as hydrogen sulfide. Also the elements
surrounding oxygen in the periodic table, nitrogen, fluorine, phosphorus, sulfur
and chlorine, all combine with hydrogen to produce gases under standard
conditions. The reason that oxygen dihydride (water) forms a liquid is that it is
more electronegative than all of these elements (other than fluorine). Oxygen
attracts electrons much more strongly than hydrogen, resulting in a net positive
charge on the hydrogen atoms, and a net negative charge on the oxygen atom.
The presence of a charge on each of these atoms gives each water molecule a
net dipole moment. Electrical attraction between water molecules due to this
dipole pulls individual molecules closer together, making it more difficult to
separate the molecules and therefore raising the boiling point. This attraction is
known as hydrogen bonding. Water can be described as a polar liquid that
dissociates disproportionately into the hydronium ion (H
3
O
+
(aq)
) and an
associated hydroxide ion (OH
−
(aq)
). Water is in dynamic equilibrium between the
liquid, gas and solid states at standard temperature and pressure (0°C, 100.000
kPa) , and is the only pure substance found naturally on Earth to be so.