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Water has many unusual
properties that make it unlike any other liquid, some of which make it
an ideal substance for aquatic habitats.
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Water molecule. One oxygen atom joined to
two hydrogen atoms - the famous H2O
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It takes a lot of energy to bring about a change in water
temperature. On a hot day pebbles on a beach will often get hot enough
to burn your feet, and it is sometimes possible to fry an egg on the
bonnet of a car that has been standing in the sun! Yet the average pond
will not increase in temperature by more than a few degrees, even on the
hottest day.
Even on cold days, water temperature changes slowly. Without this
property, water would quickly reach unacceptably high temperatures on a
hot summers day and freeze solid on a cold winters day! This resistance gives aquatic environments a high
degree of thermal stability, which means that fish, unlike ourselves and
other warm-blooded creatures, do not have to waste valuable energy
trying to maintain a constant body temperature.
Water freezes or melts at higher temperatures than its structure
would suggest (certainly, when compared with a substance of similar
molecular structure such as hydrogen sulphide, H2S). The
atomic structure of water suggests that it should be a gas at room
temperatures. Also, unlike other similar molecules, there is a large
difference in temperature between the freezing and boiling points (a
difference of 100oC). This makes water a secure habitat for
fish because it is not constantly evaporating or freezing solid.
Ice floats but why?
Most substances get denser as they get colder. For instance, on
cooling, a gas turns into a liquid and, on further cooling, the liquid
becomes a solid. Thus, substances become more dense or more solid on
cooling. Water is unusual in that it reaches its maximum density at
about 4oC and below this temperature it becomes
less dense. This has a very important consequence for outdoor fish ponds
in winter. To explain this first let me remind you that things which are
less dense - less solid - than water will float. (For example, a piece
of wood or a duck will float but a lead weight will sink.)
Now the odd thing about water is that at temperatures below 4oC
the colder water - because it is less dense - will 'float' to the top of
the pond, leaving the denser, warmer water at the bottom. Because of
this peculiarity, ice floats on water and, by forming an insulating
layer on the surface, helps prevent water below from further cooling and
thus freezing. (That is not to say that the water at the bottom of the
pond could never freeze. For a very small, shallow pond during a hard
winter it is indeed possible.) It is the warmer water at the bottom of a
pond which has enabled fish and other aquatic animals to survive during
severe winters. However, fish keepers should be aware of that pumps and
air-stones will 'mix' the water and produce a near uniform temperature
over the whole depth of the pond, particularly when bottom drain feeds
are used. Under such conditions the bottom temperature may be just above
freezing and the 'comfort zone' lost. During such spells water turnover
should be reduced to a minimum.
Water an excellent solvent
Water can dissolve many substances, including atmospheric gases like
carbon dioxide and oxygen (and fish breathe in oxygen and breathe out
carbon dioxide, just as we humans do). Indeed there are few substances
which do not dissolve in water to some extent. It is this property which
turns water into a complex chemical cocktail
and causes its composition to vary from one area of the country to
another.
Why is water so unusual?
The chemical symbol for water is H20. These symbols are a
shorthand which tell us that water consists of two atoms of hydrogen and
one atom of oxygen. These atoms are joined together to form a molecule
of water. (A molecule of water is the smallest possible bit of water,
which is invisible to the naked eye, so that water as we see it consists
of millions of water molecules). The water molecule is unusual in that
one end of it carries a slight positive electrical charge, and the other
end a negative charge, like a magnet. These charges attract water
molecules to one another and causes them to stick together.
It is this "stickiness" that gives water many of its
unusual properties and resistance to changes in temperature. It requires
a lot of energy (heat) to break these bonds and bring about a change in
temperature. In general, charged particles have only a single charge
(either positive or particle having the opposite charge (opposites
attract). Particles of the same charge will repel each other. But water,
because it has a different charge at each end, can attract both
negatively and positively charged particles. Now, for any substance to
dissolve in a liquid there has to be some attraction between the
substance and the liquid particles. The ability of water to attract
other charged particles, regardless of their charge, gives water its
superb solvent powers as well as its ability to take part in many
chemical reactions.
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Dissociation
of water molecule. Click on thumbnail to enlarge
picture
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The water molecule can also be split to form two charged particles
called ions. (An ion is a general term for a particle that is either
positively or negatively charged.) As explained above, water consists
of hydrogen and oxygen atoms combined together, which are uncharged. But
when the bond between them is broken the water molecule divides to form
hydrogen ions, H+, and hydroxyl ions, OH-. These hydrogen and hydroxyl
ions can take part in chemical reactions with other charged particles,
thereby forming new chemicals.
So we see that water is not the inert substance that we might have
imagined it to be. It has many properties which are unusual. Somewhat
paradoxically, as well as providing a stable, secure environment, it is
also a reactive substance and this enables it to take part in a
multitude of chemical and biological reactions which play such a major
part in determining the quality of the environment in a fish pond or
tank.
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