A chloride ion is much larger than a chlorine atom, 167 and 99 pm, respectively. The ion is colorless and diamagnetic. In aqueous solution, it is highly soluble in most cases; however, for some chloride salts, such as silver chloride, lead (II) chloride, and mercury(I) chloride, they are slightly soluble in water. In aqueous solution, chloride is bound by the protic end of the water molecules.
Chloride is found as an electrolyte, can flow through chloride channels (including the GABAA receptor) and is transported by KCC2 and NKCC2 transporters. Chloride is usually (though not always) at a higher extracellular concentration, causing it to have a negative reversal potential (around -61 mV at 37 degrees Celsius in a mammalian cell). Chloride is an essential electrolyte, playing a key role in maintaining cell homeostasis and transmitting action potentials in neurons. Characteristic concentrations of chloride in model organisms are: in both E. coli and budding yeast are 10-200mM (media dependent), in mammalian cell 5-100mM and in blood plasma 100mM. The concentration of chloride in the blood is called serum chloride, and this concentration is regulated by the kidneys. A chloride ion is a structural component of some proteins, e.g., it is present in the amylase enzyme. For these roles, chloride is one of the essential dietary mineral (listed by its element name chlorine).
The presence of chlorides, e.g. in seawater, significantly worsens the conditions for pitting corrosion of most metals (including stainless steels, aluminum and high-alloyed materials) by enhancing the formation, which, like most chloride salts, is colorless and water-soluble.
Calcium chloride (CaCl2) is a salt that is marketed in pellet form for removing dampness from rooms. Calcium chloride is also used for maintaining unpaved roads and for fortifying roadbases for new construction. In addition, calcium chloride is widely used as a de-icer, since it is effective in lowering the melting point when applied to ice.
A major application involving chloride is desalination, which involves the energy intensive removal of chloride salts to give potable water. In the petroleum industry, the chlorides are a closely monitored constituent of the mud system. An increase of the chlorides in the mud system may be an indication of drilling into a high-pressure saltwater formation. Its increase can also indicate the poor quality of a target sand.
Chloride is also a useful and reliable chemical indicator of river / groundwater fecal contamination, as chloride is a non-reactive solute and ubiquitous to sewage & potable water. Many water regulating companies around the world utilize chloride to check the contamination levels of the rivers and potable water sources.