By Riti Gupta, updated March 24, 2022
Everyday encounters—from squeezing lime juice on a taco to clearing a clogged drain—introduce us to acids and bases without us realizing it. Lime juice is acidic, while a typical drain cleaner contains a strong base that dissolves clogs.
But what exactly distinguishes these two classes of chemicals on a molecular level? What commonalities do they share, and how do their behaviors differ?
The most straightforward classification is the Arrhenius definition, which applies only to aqueous solutions. An Arrhenius acid raises the concentration of hydronium ions (H₃O⁺), while an Arrhenius base increases hydroxide ions (OH⁻). Classic examples are hydrochloric acid (HCl) and sodium hydroxide (NaOH), respectively.
Acids donate protons in water; bases provide hydroxide ions.
Because Arrhenius focuses solely on water, it cannot describe nonaqueous reactions. The Brønsted–Lowry framework extends the concept by emphasizing proton transfer. Any species that donates a proton is an acid; any species that accepts a proton is a base. Consequently, all Arrhenius acids and bases are also Brønsted–Lowry acids and bases.
The most inclusive description is the Lewis definition, which treats acids as electron‑pair acceptors and bases as electron‑pair donors. Under this view, an acid can form a covalent bond with a base’s electron pair, broadening the range of reactions that can be classified as acid–base.
Lewis acids accept electron pairs; Lewis bases donate them.
To determine whether a substance is acidic or basic, chemists use the pH scale, which quantifies hydrogen‑ion concentration. The pH is defined as the negative logarithm of the hydronium ion concentration:
\\(\\mathrm{pH=-log[H^+]}\\)
Similarly, the pOH is the negative logarithm of the hydroxide ion concentration:
\\(\\mathrm{pOH=-log[OH^-]}\\)
Because water is neutral when [H⁺] = [OH⁻], the relationship pH + pOH = 14 holds at 25 °C.
A pH below 7 indicates acidity, a pH of 7 denotes neutrality, and a pH above 7 signals a base. For precise measurements, a pH meter is employed; for quick assessment, the definitions above provide a reliable guide.
