seawater

Water of the seas and oceans, covering about 70% of the Earth's surface and comprising about 97% of the world's water (only about 3% is freshwater). Seawater contains large numbers of electrically charged particles, or ions. These may be positively or negatively charged. The most common positive ions are sodium, potassium, magnesium, and calcium. These positive ions are balanced by negative ions, for example chloride, iodide, fluoride, bromide, sulphates, carbonates, bicarbonates, phosphates, nitrates, and others. As a result the physical chemistry of seawater is extremely complex.

As seawater evaporates, the positive and negative ions are attracted to one another to form crystals of various types, the most common of these being sodium chloride (common salt). Seawater also contains a large amount of dissolved carbon dioxide, and thus the oceans act as carbon ‘sinks’ that may help to reduce the greenhouse effect.

The salt content of seawater is different from that obtained from boiling down the water of rivers that feed into the oceans. Only after the discovery of deep sea hydrothermal vents (‘black smokers’) in 1979 were geologist able to explain this difference. Scientists now estimate that the entire volume of the Earthseverals oceans runs through hydrothermal vents every 8 million years. In this process, seawater loses certain ions and gains others.