Hydrochloric acid (HCl) is a compound, not a mixture. It consists of hydrogen and chlorine atoms that are chemically bonded together in a fixed ratio. In a compound, the elements are combined in such a way that they form a new substance with distinct properties that are different from those of the individual elements.
Mixtures, on the other hand, are combinations of two or more substances where each retains its own properties. The components can be separated by physical means, and they do not change chemically when mixed. Since hydrochloric acid has a specific formula, HCl, and exhibits consistent behavior in chemical reactions, it confirms its identity as a compound.