The liquid product commonly referred to as “hydrochloric acid” is chemically classified as a mixture. However, the molecule that forms the basis of this acid, hydrogen chloride (\(\text{HCl}\)), is a pure substance known as a compound. Understanding this distinction requires examining the fundamental differences between these two categories of matter.
Pure Substances Versus Mixtures
Pure substances are defined by a uniform and constant composition throughout. These substances fall into two categories: elements, made up of only one type of atom, and compounds, which contain two or more different elements chemically bonded together in a fixed ratio. A compound cannot be separated into its constituent elements by physical methods, requiring a chemical reaction instead.
Mixtures, in contrast, consist of two or more pure substances that are physically combined. The components retain their individual chemical properties, and their ratio can vary widely. For example, a mixture like salt water can be separated back into its components by boiling the water away. Mixtures are categorized as either heterogeneous (components visibly separate) or homogeneous (composition uniform throughout, such as a solution).
The Structure of Hydrogen Chloride
The chemical entity represented by the formula \(\text{HCl}\) is hydrogen chloride, a compound that exists as a colorless gas. This molecule is a pure substance because it consists of one hydrogen atom and one chlorine atom joined by a single covalent bond. Since the bond ratio is fixed at one-to-one, the composition of the molecule is constant.
The bond involves the sharing of electrons, resulting in a new chemical species with properties different from elemental hydrogen and chlorine. Because this chemical combination cannot be separated by physical processes like filtration or evaporation, hydrogen chloride gas is classified as a compound. The \(\text{HCl}\) molecule is highly polar because the chlorine atom attracts the shared electrons more strongly than the hydrogen atom.
Why Hydrochloric Acid is a Solution
The liquid commonly known as hydrochloric acid is the aqueous solution of hydrogen chloride gas, often represented by the notation \(\text{HCl}(\text{aq})\). This liquid is created by dissolving the pure hydrogen chloride gas into water. Chemically, a solution is a type of homogeneous mixture.
When the \(\text{HCl}\) gas dissolves in water, the molecules dissociate into hydrogen ions (\(\text{H}^+\)) and chloride ions (\(\text{Cl}^-\)). The presence of these separated ions gives the liquid its characteristic acidic properties. Since the liquid contains two distinct, non-chemically bonded substances—dissolved hydrogen chloride and water—it meets the definition of a mixture.
The concentration of hydrochloric acid can be varied, which is a defining trait of a mixture. Commercial preparations are commonly sold at different concentrations, often ranging from 10% to over 30% hydrogen chloride by mass. This variable ratio confirms that the liquid hydrochloric acid is a homogeneous mixture.