Theories of Acids and Bases

In 1884, Svante Arrhenius proposed that acidity is all about what happens in water. He defined an acid as a substance that increases the concentration of hydrogen ions ($H^+$) in aqueous solution, and a base as a substance that increases hydroxide ions ($OH^-$). While groundbreaking, this theory was limited: it only applied to reactions in water and couldn't explain why substances like ammonia ($NH_3$) act as bases despite having no $OH^-$ in their formula. We now view Arrhenius's work as a specific subset of a much broader chemical truth.

In 1923, Johannes Brønsted and Thomas Lowry independently expanded the definition. They shifted the focus from 'what ions are produced' to 'where the protons go.' In this model, an acid is a proton donor, and a base is a proton acceptor. A 'proton' is simply a hydrogen atom that has lost its electron ($H^+$). This definition is universal—it applies to gases, solids, and non-aqueous liquids. If a molecule has an $H$ to give, it's a potential acid; if it has a lone pair of electrons to grab an $H$, it's a potential base.

Chemical reactions are reversible. When a Bronsted-Lowry acid loses a proton, the remaining piece is now capable of taking that proton back—meaning it has become a conjugate base. Conversely, when a base accepts a proton, it becomes a conjugate acid. These two species, which differ only by a single $H^+$, are called a conjugate acid-base pair. Identifying these pairs is like tracking a baton in a relay race: the acid holds the baton (proton), and the conjugate base is the runner after the hand-off.

Some substances are chemical 'chameleons.' An amphiprotic substance is a molecule or ion that can either donate or accept a proton depending on what it is reacting with. Water ($H_2O$) is the most common example. If water reacts with a strong acid, it acts as a base. If it reacts with a strong base, it acts as an acid. Other examples include ions from polyprotic acids, such as bicarbonate ($HCO_3^-$) or bisulfate ($HSO_4^-$). Their behavior is dictated entirely by their environment.