When one acid is mixed with another, the outcome can range from simple physical mixing to complex chemical reactions. Acids are substances defined by their ability to donate a proton (hydrogen ion), which increases acidity when dissolved in a solvent like water. Since both components are acids, the interaction is not a simple neutralization reaction. The outcome depends entirely on the specific chemical properties of the two acids involved.
Defining the Difference: Dissolution versus Chemical Reaction
Dissolution is primarily a physical change where one substance, the solute, disperses evenly throughout another substance, the solvent, to form a homogeneous mixture. The original molecules or ions remain structurally intact and can theoretically be separated by physical means, such as evaporating the solvent.
In contrast, a chemical reaction involves the breaking of existing chemical bonds and the formation of entirely new bonds, transforming starting materials (reactants) into new substances (products). When acids dissolve in water, the acid molecule reacts with water to produce hydronium ions, which is technically a chemical reaction called ionization. However, when two already-ionized acids are mixed, the primary interaction is often closer to physical mixing.
The Standard Outcome: Mixing and Ionization
The most common result when two typical aqueous acids, such as hydrochloric acid (\(\text{HCl}\)) and sulfuric acid (\(\text{H}_2\text{SO}_4\)), are mixed is simple dissolution and a combined concentration of ions. Both acids are already dissolved in water and exist as separate ions, including the shared hydrogen ion (\(\text{H}^+\) or \(\text{H}_3\text{O}^+\)). Mixing them simply combines these existing ions into a larger solution.
No new molecular compounds are generated from the interaction between the two acids; the mixture becomes more acidic than either component alone. The total concentration of the active hydrogen ions in the resulting solution is the sum of the hydrogen ions contributed by each individual acid. This increased concentration results in a lower \(\text{pH}\) value.
The presence of the common hydrogen ion can influence the behavior of a weaker acid in the mixture, a phenomenon known as the common ion effect. If a strong acid is mixed with a weak acid, the high concentration of hydrogen ions from the strong acid will suppress the ionization of the weaker acid. This means that the weak acid will dissociate less than it would in pure water, but the overall result remains a mixture of ions with no new compound formation.
When Reactions Occur: Special Acid Interactions
While simple mixing is the default outcome, a true chemical reaction between two acids can occur if one acid possesses a specific chemical property that allows it to chemically alter the other. This transformation often involves oxidation-reduction (redox) reactions, where electrons are transferred between the components.
The most famous example is the formation of aqua regia, a highly corrosive mixture of concentrated nitric acid (\(\text{HNO}_3\)) and hydrochloric acid (\(\text{HCl}\)). In this specific mixture, the nitric acid acts as a powerful oxidizing agent, chemically reacting with the hydrochloric acid to produce new, volatile compounds. The reaction yields nitrosyl chloride (\(\text{NOCl}\)), chlorine gas (\(\text{Cl}_2\)), and water. The production of these new chemicals, particularly the highly reactive chlorine gas, is what allows aqua regia to dissolve noble metals like gold, which neither acid can accomplish alone.
Another scenario for acid-acid reaction involves complex formation, where the anion of one acid can form a highly stable, non-dissociated complex with a component of the other. These reactions are transformations driven by the strong affinity between the two chemical species. The occurrence of these reactions is an exception based on the unique oxidizing power or complexing ability of the acids involved.