Why seawater composition does not change?
Just as the salt content of seawater does not change, the concentration of
its components also remains unchanged. To maintain the constant
composition, it is necessary that dissolved ions are removed at the same
rate as they are added. Processes by which ions are removed from
seawater include the following:
- Evaporative precipitation: In hot, dry climates where the sea is shallow
and enclosed, solid deposits of salts such as rock salt (sodium chloride)
and gypsum (calcium sulphate) are formed.
- Chemical precipitation: When the concentration of a salt becomes too
great, that salt forms a solid precipitate. For example, calcium ions and
carbonate ions combine together to form insoluble limestone (calcium
carbonate).
- Biochemical removal: Organisms remove ions by scavenging them from
seawater. Some animals like coral and bivalves make shells of calcium
carbonate. Many organisms concentrate ions in their body tissues by a
factor of 105 or more. For example, sea squirts concentrate vanadium,
other tunicates concentrate niobium, oysters concentrate zinc, lobsters
concentrate copper, and other shellfish concentrate mercury.
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