The principle substance in seawater is water, two hydrogen atoms covalently
bonded to one oxygen atom, forming a highly polar molecule by the unequal
distribution of the shared electrons and resulting in a negative pole at the
oxygen atom and two positive poles at the hydrogen atoms. It is this
polarity that makes water so unique and gives it a great ability to act as
Because water is a powerful solvent, a great number of materials found in
seawater are dissolved, and thus exist in their ionic forms. The most
abundant ions in seawater are: chloride, sodium, sulfate, magnesium,
calcium, potassium, and bicarbonate. These ions, along with the hydrogen and
oxygen of water, make up 99.9% of the components of seawater.
Taken as a whole, these ions make up the salinity of seawater, which is 3.7%
by weight. These ions have some very interesting effects on seawater, which
account for the distribution of the life forms inhabiting the oceans.
These effects, known as colligative properties, include: lowering
of heat needed to raise the temperature of seawater, lowering the freezing
point, slowing the evaporation, and increasing osmotic pressure with
With new and more sensitive techniques for the analysis of seawater,
eighty elements, some with concentrations below 1 part per billion
(1 mg per 1000 kg), have been detected in seawater. However, the ratio of
the major salts (listed above) remains constant throughout the oceans,
allowing scientists to measure salinity simply by measuring the amount of
one element in seawater; the element of choice is chlorine.
An interesting observation is that the seas have not gotten progressively
saltier with age; in fact, the basic composition of seawater has remained
constant for about one billion years. This indicates a state of equilibrium
in the ocean; a sort-of steady-state condition. However, since there is a
continuous input of ions into the oceans, there must also be a similar
output, or removal, of ions from the oceans. The time it takes for an ion
to enter and then leave the ocean is its residence time, which can vary from
100 million years for chlorine to 200 years for iron. This factor is also
a measure of the reactivity of the element in the ocean, with shorter
residence times indicating more reactive elements.
Gases are also dissolved in water, with nitrogen, oxygen, and carbon
the most abundant and important. Nitrogen affects the production of
by ocean-dwelling organisms, oxygen is used by the vast majority of
lifeforms in the oceans, and carbon dioxide is used by plants during
photosynthesis and also affects the acid-base balance in seawater by
forming bicarbonate and carbonate ions under varying conditions. These
interactions can become quite complex, and have major impacts on the
distribution of life in the oceans.
This completes What is in Seawater?
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