Hydrochloric acid or hydrochloric acid (salt ink) is a clear, colorless, and highly odorous solution of hydrogen chloride in water with the chemical formula HCl. It is a highly corrosive and acidic mineral acid with many industrial applications. This acid is widely used because it is one of the basic chemicals. Colorless hydrochloric acid gas has a pungent odor, an acidic taste, and is heavier than air. It is easily liquefied and, when wet or dissolved, turns blue to red. Hydrochloric acid has long been known as salicylic acid, muriatic acid, and salt spirits, and is derived from vitriol (sulfuric acid) and common salt. Free hydrochloric acid was first officially introduced in the 16th century by Andreas Libavius. After him, chemists such as Johann Rudolf Glaber, Joseph Priestley, and Humphrey Davy used the substance in their scientific research.

With the onset of mass production in the Industrial Revolution, hydrochloric acid was used in the chemical industry as a unique reagent in the production of vinyl chloride for PVC plastics and MDI / TDI for the production of polyurethane. It also has many small-scale applications that can be used for housekeeping, gelatin making, descaling, leather making and food additives. Hydrochloric acid is also produced as a by-product of the production of toluene diisocyanate (TDI). About 20 million tons of hydrochloric acid are produced annually in the world.

Hydrochloric acid is a basic chemical and therefore has a wide range of uses. Hydrochloric acid has an acidic and corrosive gas.

Hydrochloric acid gas is colorless with a pungent gas odor and has an acidic taste and is heavier than air. It easily liquefies to form a colorless liquid, and when wet or soluble, it turns the blue substance red. It combines with metals and metal oxides and their hydroxides.