Which compound is known as hydrochloric acid when dissolved in water?

Hydrogen chloride is the compound known as hydrochloric acid when dissolved in water.

Hydrogen chloride has the formula HCl. In its pure form, HCl is a colorless gas with a sharp, irritating odor. When HCl dissolves in water, it ionizes extensively, producing hydronium and chloride ions; that aqueous solution is called hydrochloric acid.

This distinction matters because “hydrogen chloride” refers to the compound itself, while “hydrochloric acid” refers specifically to its water solution. Concentrated hydrochloric acid is strongly corrosive and is used in laboratory work, metal processing, and industrial chemistry.

Hydrogen chloride should not be confused with sodium chloride, table salt. Both contain chlorine, but sodium chloride is an ionic solid made from sodium and chloride ions, whereas HCl contains hydrogen and chlorine.