The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Cl⁻. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. An example is table salt, which is sodium chloride with the chemical formula NaCl. In water, it dissolves into Na⁺ and Cl⁻ ions.

The word chloride can also refer to a chemical compound in which one or more chlorine atoms are covalently bonded in the molecule. This means that chlorides can be either inorganic or organic compounds. The simplest example of an inorganic covalently-bonded chloride is hydrogen chloride, HCl. A simple example of an organic covalently-bonded (an organochloride) chloride is chloromethane (CH₃Cl), often called methyl chloride.

In the petroleum industry, the chlorides are a closely monitored constituent of the mud system. The increase of the chlorides in the mud system could indicate the possibility of drilling into a high-pressure saltwater formation. Its increase can also indicate the poor quality of a target sand.

Other examples of inorganic covalently-bonded chlorides that are used as reactants are:

• phosphorus trichloride, phosphorus pentachloride, and thionyl chloride, all three of which reactive chlorinating reagents that have been used in a laboratory
• disulfur dichloride (S₂Cl₂), used for vulcanization of rubber.

Human health

Chloride is a chemical the human body needs for metabolism (the process of turning food into energy). It also helps keep the body's acid-base balance. The amount of chloride in the blood is carefully controlled by the kidneys. Further reading:Renal chloride reabsorption

Chloride ions have important physiological roles. For instance, in the central nervous system, the inhibitory action of glycine and some of the action of GABA relies on the entry of Cl⁻ into specific neurons. Also, the chloride-bicarbonate exchanger biological transport protein relies on the chloride ion to increase the blood's capacity of carbon dioxide, in the form of the bicarbonate ion.

The normal blood reference range of chloride for adults in most labs is 95 to 105 milliequivalents (mEq) per liter. The normal range may vary slightly from lab to lab. Normal ranges are usually shown next to your results in the lab report.

The North American Dietary Reference Intake recommends a daily intake of between 2300 and 3600 mg/day for 25-year-old males.

Other applications

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.