Hydrogen chloride is a chemical compound with the formula \(\text{HCl}\). The substance exists as a colorless gas at room temperature, but its most recognized form is when it dissolves in water to create hydrochloric acid. This aqueous solution is a strong, corrosive acid that plays a significant role in both industrial processes and human biology.
What Defines a Chemical Compound
A chemical compound is a substance formed when atoms from two or more different elements are chemically bonded together. A compound must also adhere to the Law of Definite Proportions, meaning the elements are always present in fixed, definite mass ratios.
This definition sets compounds apart from pure elements, which consist of only one type of atom, such as elemental hydrogen (\(\text{H}_2\)) or chlorine (\(\text{Cl}_2\)). It also distinguishes them from mixtures, where components are physically combined and can be separated without a chemical reaction.
The Fixed Ratio of Hydrogen and Chlorine
Hydrogen chloride’s status as a compound is confirmed by its precise molecular structure. Each molecule is formed from one atom of hydrogen (\(\text{H}\)) and one atom of chlorine (\(\text{Cl}\)).
These two atoms are held together by a single polar covalent bond. While a covalent bond involves sharing electrons, in \(\text{HCl}\), the sharing is unequal. Chlorine has a much higher electronegativity than hydrogen, pulling the shared electrons closer to its nucleus.
This uneven distribution gives the chlorine atom a slight negative partial charge (\(\delta-\)) and the hydrogen atom a slight positive partial charge (\(\delta+\)). The resulting polarity contributes to its high solubility in water. Because the bond involves electron sharing, gaseous hydrogen chloride is classified as a molecular compound.
Hydrogen Chloride in Practical Use
While gaseous hydrogen chloride is the pure compound, its industrial and biological importance stems almost entirely from its aqueous solution, hydrochloric acid. The acidic properties of this solution are harnessed across various fields.
A significant industrial application is the “pickling” of steel, a process that removes rust and scale from the metal surface. Hydrochloric acid is also a reagent in chemical manufacturing, used in the production of vinyl chloride, the precursor for the plastic polyvinyl chloride (\(\text{PVC}\)). It serves a function in water treatment by regenerating ion-exchange resins used to purify water.
In the human body, a dilute solution of hydrochloric acid is naturally produced by the stomach lining. This strong acid, often referred to as gastric acid, is necessary for digestion. It helps break down complex foods and activates the enzyme pepsin, while also acting as a defense mechanism by killing most bacteria ingested with food.