Hydrochloric acid has the chemical formula HCl. It is an aqueous solution of hydrogen chloride, which when in a saturated form contains about 43% hydrogen chloride. Hydrogen chloride in aqueous solution is a strong electrolyte because it disassociates to form ions, H+ and Cl-. This disassociation is complete so no molecules of HCl are found. Hydrochloric acid is a proton donor, this is the hydrogen ion. When ionization occurs only one hydrogen ion is produced per molecule of HCl, which is an example of a monoprotic acid. Because ionization is so complete with hydrochloric acid it is regarded as a strong acid and the maximum number of hydrogen ions are freed up per molecule. Hydrochloric acid is used in a wide range of chemical reactions and conversions.
Hydrochloric acid can be used to clean metal surfaces and can be found naturally occurring in the stomachs of many animals as a relatively weak solution. Excess hydrochloric acid in the stomach can cause dyspepsia. It is also commonly encountered in the food industry. It is an extremely corrosive liquid and must be stored and handled appropriately. Because hydrochloric acid does not contain any organic molecules it is regarded as a mineral acid.
Hydrogen chloride is prepared by reacting sodium chloride (common table salt) with concentrated sulfuric acid. The products of this reaction are solid sodium hydrogen sulfide and gaseous hydrogen chloride. This gaseous hydrogen chloride can then be dissolved in water, by bubbling it through the water, to give hydrochloric acid. In the 1990s the annual United States production of hydrochloric acid was around 3 megatonnes.
The properties of hydrochloric acid are typical of any strong mineral acid. Hydrochloric acid is a corrosive substance, as such it can be used to clean metal surfaces. Any dirt or oxide is eaten away by the acid exposing pure metal. Hydrochloric acid when manufactured as explained earlier can have a pH value of 0.0, when encountered in the stomach the pH value is closer to 1.0, one tenth the strength. Hydrogen chloride as a gas is not acidic itself. It is a covalent compound and for acidic properties to be shown the molecules have to disassociate to the separate ions. For example if hydrogen chloride is dissolved in an organic compound such as methylbenzene no acidic properties are shown, unlike when it is dissolved in water.
When hydrochloric acid reacts with a base the salt of the base is produced. An example of this ispotassium hydroxide and hydrochloric acid reacting together to give potassium chloride. Hydrochloric acid will neutralize a base in a chemical reaction. Hydrochloric acid reacted with a metal will dissolve the outer layer of the metal, cleaning it and evolving hydrogen gas in the process as well as producing the chloride salt of the metal. Some unreactive metals react only very slowly, such as copper and lead. More reactive metals such as sodium or calcium react very violently and it is not recommended to try these reactions. Hydrochloric acid will react with a chloride to produce carbon dioxide, water, and the salt of the chloride. For example calcium carbonate (chalk) will give water, carbon dioxide, and calcium chloride.
Hydrochloric acid is a typical mineral acid, undergoing all standard acid reactions. Because disassociation in water is so complete hydrochloric acid is a strong electrolyte, proton donor, and acid.
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