Hydrochloric Acid (HCl) — Properties, Uses & Safety

Overview

Hydrochloric acid (HCl) is a strong monoprotic acid formed when hydrogen chloride gas dissolves in water. With a molar mass of 36.46 g/mol, it is one of the most fundamental acids in chemistry and industry. Concentrated hydrochloric acid (about 37% HCl by mass) is a colorless to slightly yellow fuming liquid with a sharp, pungent odor. As a strong acid, HCl dissociates completely in water (HCl → H⁺ + Cl⁻), making it an ideal example for teaching strong acid behavior, pH calculations, and stoichiometry. Hydrochloric acid occurs naturally in the human body as the primary component of gastric acid (stomach acid), where it is secreted by parietal cells at a concentration of about 0.5% (pH 1-2). Gastric HCl activates the digestive enzyme pepsin, kills ingested bacteria, and helps denature proteins for digestion. Industrially, hydrochloric acid is produced as a byproduct of chlorination reactions in organic chemistry or by the direct synthesis of hydrogen and chlorine gases. It is one of the least hazardous strong acids to handle because the chloride ion is relatively non-toxic and non-reactive. Major industrial uses include steel pickling (removing rust and scale from metal surfaces), production of organic compounds (vinyl chloride for PVC), pH regulation in water treatment and swimming pools, food processing (gelatin production, corn syrup), oil well acidizing in petroleum extraction, and as a laboratory reagent for titrations and qualitative analysis. Hydrochloric acid is central to many classic chemistry experiments. The reaction of HCl with sodium hydroxide (HCl + NaOH → NaCl + H2O) is the prototypical acid-base neutralization reaction. The reaction with metals like zinc (Zn + 2 HCl → ZnCl2 + H2) demonstrates single displacement reactions and hydrogen gas production. The reaction with carbonates (CaCO3 + 2 HCl → CaCl2 + H2O + CO2) is used to test for carbonate minerals. Safety considerations include its corrosive nature — concentrated HCl can cause severe burns to skin and eyes, and its fumes are irritating to the respiratory tract. GHS pictograms include the corrosion and exclamation mark symbols. For chemistry students, hydrochloric acid is indispensable for learning about strong acid dissociation, molarity and dilution calculations, titration techniques, acid-base indicators, neutralization reactions, and the activity series of metals. It is one of the first acids students encounter and remains relevant throughout their chemistry education.

IUPAC Name

hydrochloric acid

CAS Number

7647-01-0

PubChem CID

313

Molecular Formula

HCl

Molar Mass

36.461 g/mol

State at STP

gas

Appearance

Colorless to slightly yellow gas; aqueous solution is fuming liquid