By Robert Allen – Updated Mar 24, 2022
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Hydrochloric acid (HCl) forms when hydrogen chloride gas dissolves in water, reaching concentrations up to about 40 % by weight. While it reacts with a wide range of compounds, its behavior with metals—especially those on the left side of the periodic table—is particularly notable.
HCl readily attacks most metals except those in the platinum group. Reactivity decreases from left to right across the table, with alkali metals reacting the fastest and platinum‑group metals remaining inert unless treated with aqua regia.
The first‑group elements—lithium, sodium, and potassium—react vigorously with even cold water, producing hydroxides and hydrogen gas. In the presence of HCl, they yield the corresponding chloride salts and hydrogen. For example, sodium reacts with two molecules of HCl to form two molecules of sodium chloride (table salt) and one molecule of H₂.
Elements in the second group (beryllium, magnesium, calcium, strontium) are less reactive than alkali metals but still dissolve in HCl, forming their respective chlorides and releasing hydrogen gas. Magnesium chloride, produced in this way, is commonly used as a dietary supplement, while calcium and strontium chlorides find industrial applications.
Iron, cadmium, cobalt, nickel, tin, and lead do not react with water but are soluble in HCl. The reaction displaces hydrogen, yielding metal chlorides: iron gives FeCl₂ (ferrous chloride), used in wastewater treatment to precipitate suspended particles; cadmium, cobalt, nickel, and tin chlorides are key electrolytes in electroplating processes.
Metals such as platinum, palladium, rhodium, ruthenium, iridium, and gold are largely inert to HCl alone. However, when combined with nitric acid to form aqua regia—Latin for “royal water”—they dissolve readily. Aqua regia is the industry standard for refining gold and silver to high purity and for cleaning laboratory glassware that has become contaminated with any metal residue.
Although HCl alone cannot attack the most noble metals, its mixture with nitric acid creates an exceptionally corrosive solution capable of dissolving even “royal” metals. This property is exploited by metal refiners and chemists alike, ensuring the production of ultra‑pure metals for investment coins and the removal of contaminants from lab equipment.
