By Chris Deziel – Updated Aug 30, 2022
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In water, acids and bases neutralize to form salts. In nonaqueous settings, the products may still be salts or complex ions, depending on the theory applied.
Svante Arrhenius first described acids as substances that release H⁺ ions in aqueous solution, forming hydronium ions (H₃O⁺). The concentration of these ions defines the pH; lower pH values indicate higher acidity. Bases, by contrast, supply hydroxide ions (OH⁻), raising the pH above 7 and rendering the solution alkaline. Classic examples are hydrochloric acid (HCl) and sodium hydroxide (NaOH).
When an Arrhenius acid and base meet, the hydronium and hydroxide ions combine to produce water, and the remaining cations and anions pair up to form a salt. This process, known as hydrolysis, brings the solution to a neutral pH. A textbook illustration is the reaction of HCl with NaOH to yield sodium chloride (NaCl).
Brønsted and Lowry broadened the definition to any species that donates a proton (H⁺) or accepts one. This framework explains reactions beyond aqueous solutions, such as the neutralization of ammonia (NH₃) by HCl to produce ammonium chloride (NH₄Cl). Although no hydronium or hydroxide ions are involved, the result is still a salt.
G.N. Lewis introduced an even more inclusive view, defining acids as electron‑pair acceptors and bases as electron‑pair donors. This model accounts for reactions that cannot be described by proton transfer, including interactions between gases, liquids, and solids. In the Lewis framework, acid‑base reactions can yield non‑salt products, such as the complex ion tetraamminezinc:
Zn²⁺ + 4 NH₃ → [Zn(NH₃)₄]⁴⁺
Thus, the outcome of combining an acid and a base depends on the chosen theoretical lens: neutralization to water and a salt, or formation of a complex ion.
