Learn about salinity and related concepts

Learn about salinity and water quality

Useful vocabulary

Ion: cation, anion

Calcium ion Ca++
Iron ions Fe++
Magnesium ion Mg++
Sodium ion Na+

Minerals eventually decompose and may become dissolved in water as a result of weathering — the disintegration and decomposition of rock at Earth's surface.

Remember that minerals are themselves composed of atoms, the smallest complete chemical units. When freed from minerals by weathering and present in water, atoms carry an electrical charge. These charged atoms are known as ions. Positively charged ions, such as calcium, magnesium, and sodium, are called cations. Negatively charged ions, such as chloride, are called anions.

Each element released into water by the weathering process exists in its own particular charge configuration(s). Chlorine, for example, always forms chloride ion, which carries a single unit of negative charge.

Chloride ion Cl-
Carbonate CO3--
Bicarbonate HCO3-
Sulfate SO4--
Nitrate NO3-
Dissolved silica SiO2

Calcium always exists in solution with two units of positive charge. Iron, symbolized Fe, can form ions that carry two or three units of positive charge.

Atoms of some elements tend to group together in water with oxygen or hydrogen. For example, carbon nearly always exists as carbonate, consisting of one carbon atom bonded to three oxygen atoms, or as bicarbonate, which consists of one hydrogen, a carbon, and three oxygens.

Other important ion groups include:

  • Sulfate — sulfur bonded to four oxygen atoms
  • Nitrate — nitrogen plus three oxygen atoms
  • Silica — silicon bonded to two oxygen atoms
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