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Water
is a chemical compound and
polar molecule, which is
liquid under STP
conditions. It has the chemical formula H2O, meaning that one
molecule of water is composed
of two hydrogen atoms and one oxygen atom. Water is found almost
everywhere on earth and is required by all known
life.
About 70% of Earth's surface is covered by water.
The
solid state of water is known
as ice; the gaseous
state is known as water vapor (or steam).
The units of temperature (formerly the degree Celsius and now the Kelvin) are defined in terms of
the triple point of water, 273.16
K (0.01 °C) and 611.2 Pa, the temperature and pressure at which solid, liquid,
and gaseous water coexist in equilibrium. Water exhibits some very strange behaviors,
including the formation of states such as vitreous
ice—a noncrystalline (glassy), solid state of water. At
temperatures greater than 647 K and pressures greater than 22.064
MPa, a collection of water molecules
assumes a supercritical condition, in which liquid-like clusters float
within a vapor-like phase. Body
of water is a term for an ocean, sea, lake, river, stream, canal,
pond,
or the like. See water (resource)s for
information about fresh water supplies. Also see: sea water,
fresh water, and underwater. Earth's
approximate water volume (the total water supply of the world) is 326,000,000
cubic miles. Of this volume: - 316,900,000
is in the oceans
- 6,000,000
is in glaciers and icecaps
- 3,000,000
is in underground storage in aquifers
- 60,000
is in fresh water in lakes, inland seas, and rivers.
- 3,100
is water vapor at any one time.
An
important feature of water is its polar nature. The water
molecule forms an angle, with hydrogen atoms at the tips and oxygen at the vertex.
Since oxygen has a higher electronegativity than
hydrogen, the side of the molecule with the oxygen atom has a partial negative
charge, relative to the hydrogen side. A molecule with such a charge difference
is called a dipole. The charge differences
cause water molecules to be attracted to each other (the relatively positive areas
being attracted to the relatively negative areas) and to other polar molecules.
This attraction is known as hydrogen bonding.
This relatively weak (relative
to the covalent bonds within the water molecule itself) attraction results in
physical properties such as a very high boiling point, because a
lot of heat energy is necessary to break
the hydrogen bonds between molecules. For example, Sulphur is the element below
oxygen in the periodic table, and its equivalent compound, hydrogen sulphide (H2S)
does not have hydrogen bonds, and though it has twice the molecular weight of
water, it is a gas at room temperature. The extra bonding between water molecules
also gives liquid water a large specific heat capacity.
Hydrogen bonding
also gives water an unusual behaviour when freezing. Just like most other materials,
the liquid becomes denser with lowering temperature. However, unlike most other
materials, when cooled to near freezing point, the presence of hydrogen bonds
means that the molecules, as they rearrange to minimise their energy, form a structure
that is actually of lower density: hence the solid form, ice, will float in water
i.e. water expands as it freezes (most other materials shrink on solidification).
Liquid water reaches its highest density at a temperature of 4 °C. This has an
interesting consequence for water life in winter. Water chilled at the surface
becomes denser and sinks, forming convection currents that cool the whole water
body, but when the temperature of the lake water reaches 4°C, water on the surface,
as it chills further, becomes less dense, and stays as a surface layer
which eventually forms ice. Since downward convection of colder water is blocked
by the density change, any large body of water frozen in winter will have the
bulk of its water still liquid at 4°C beneath the icy surface, allowing fish to
survive. (this is one of the principal examples of finely-tuned physical properties
that support life on Earth that is used as an argument for the Anthropic Cosmological
Principle). Another
consequence is that ice will melt if sufficient pressure is applied.
Water
is also a good solvent due to its polarity. The
solvent properties of water are vital in biology, because many biochemical
reactions take place only within aqueous solutions (e.g.,
reactions in the cytoplasm and blood).
In addition, water is used to transport biological
molecules. When
an ionic or polar compound enters water, it is surrounded by water molecules.
The relatively small size of water molecules typically allows many water molecules
to surround one molecule of solute. The partially negative dipoles of
the water are attracted to positively charged components of the solute, and vice
versa for the positive dipoles. In
general, ionic and polar substances such as acids, alcohols, and salts are
easily soluble in water, and nonpolar substances such as fats and oils are not.
Nonpolar molecules stay together in water because it is energetically more favorable
for the water molecules to hydrogen bond to each other than to engage in van der Waals interactions
with nonpolar molecules. An
example of an ionic solute is table salt; the sodium
chloride, NaCl, separates into Na+ cations and Cl- anions,
each being surrounded by water molecules. The ions are then easily transported
away from their crystalline lattice into solution. An example of a nonionic solute
is table sugar. The water dipoles
hydrogen bond to the dipolar regions of the sugar molecule and allow it to be
carried away into solution. The
strong hydrogen bonds give water a high cohesiveness and, consequently, surface tension. This
is evident when small quantities of water are put onto a nonsoluble surface and
the water stays together as drops. This feature is important when water is carried
through xylem up stems in plants; the strong
intermolecular attractions hold the water column together, and prevent tension
caused by transpiration pull.
Other liquids with lower surface tension would have a higher tendency to "rip",
forming vacuum or air pockets and rendering the xylem vessel inoperative.
Pure
water is actually an insulator, meaning that it does
not conduct electricity well. Because
water is such a good solvent, it often has some solute dissolved in it, most frequently
salt. If water has such impurities, then it can conduct electricity well.
Water
can be split into its constituent elements, hydrogen and oxygen, by passing a
current through it. This process is called electrolysis. Water molecules
naturally disassociate into H+ and OH- ions, which are pulled
toward the cathode and anode, respectively.
At the cathode, two H+ ions pick up electrons and form H2
gas. At the anode, four OH- ions combine and release O2
gas, molecular water, and four electrons. The gases produced bubble to the surface,
where they can be collected.
Chemically, water is amphoteric:
able to act as an acid or base. Occasionally the term hydroxic acid is
used when water acts as an acid in a chemical reaction. At a pH of 7 (neutral),
the concentration of hydroxide ions (OH-)
is equal to that of the hydronium (H3O+)
or hydrogen ions (H+)
ions. If the equilibrium is disturbed,
the solution becomes acidic (higher concentration of hydronium ions) or basic
(higher concentration of hydroxide ions). |
