In chemistry, a base is a substance that can accept a proton, donate an electron pair, or produce hydroxide ions in water, depending on the definition used. These descriptions come from different scientific models, so the word does not have just one meaning. The Brønsted–Lowry model describes ammonia accepting a proton from water. The resulting mixture contains ammonium and hydroxide ions. Under the Arrhenius model, sodium hydroxide is a base because it increases hydroxide ions in aqueous solution. The Lewis model is broader still. That distinction matters.
Bases can be grouped in several useful ways. Strong and weak describe how extensively a base reacts or ionizes in water; they do not simply mean concentrated and dilute. Sodium hydroxide is a strong base, while ammonia is a weak one. Bases may also be classified by composition, such as inorganic metal hydroxides or organic compounds that accept protons. Another grouping counts how many protons a base can accept per formula unit. These categories help explain reactions, compare substances, and interpret laboratory results. Yet real examples do not always fit a tidy chart. Context matters: water, concentration, and the reacting partner can affect observed behavior. This guide introduces the main definitions and types of bases, with practical examples and careful distinctions. Even familiar labels deserve a second look. A clear understanding begins by asking which definition is being used.
A base can be defined by how it behaves, not just by what it contains. In the Brønsted–Lowry model, a base accepts a proton, written as H⁺, from another substance. Ammonia, for example, accepts a proton from water and forms ammonium ions. Not every base contains hydroxide. In the Arrhenius model, however, a base increases the concentration of hydroxide ions, OH⁻, in water. Sodium hydroxide does this directly, while ammonia produces hydroxide through its reaction with water. These definitions describe related but different chemical behaviors. That distinction matters.
The pH scale helps show how acidic or basic an aqueous solution is. At about 25°C, a pH above 7 is generally basic, while a value below 7 is acidic. A neutral solution is near 7. Each whole-number step represents a tenfold change in hydrogen-ion activity, so pH 10 is not merely “a little” higher than pH 9. A damp test strip changing color can offer a quick estimate, but it cannot identify every substance in a sample. The classroom rule that bases feel slippery is also a poor test; touch is neither precise nor safe. pH can shift with temperature and concentration, so a number needs context. A base’s proton-accepting ability, hydroxide ions, and measured pH are connected, but they are not interchangeable descriptions.