Base (chemistry)
Substance that reacts with acids; accepts protons or donates electrons.
In chemistry, a base is a substance that reacts with acids, a concept originally proposed by G.-F. Rouelle in the mid-18th century. Three definitions are commonly used: Arrhenius bases, Brønsted bases, and Lewis bases, each describing bases as substances that accept protons or donate electron pairs. The Arrhenius definition, proposed in 1884, describes a base as a substance that dissociates in water to produce hydroxide ions (OH⁻), such as metal hydroxides like sodium hydroxide or calcium hydroxide. These aqueous solutions have characteristic properties: they feel slippery, taste bitter, and change the color of pH indicators, for instance turning red litmus paper blue. In water, bases lower the hydrogen ion activity, yielding a pH above 7.0 at standard conditions. A soluble base that releases hydroxide ions quantitatively is called an alkali. Metal oxides, hydroxides, and alkoxides are basic, and the conjugate bases of weak acids are weak bases. The Brønsted–Lowry theory, from 1923, broadens the definition: a base is any substance that can accept a hydrogen cation (proton). This includes Arrhenius bases but also substances like ammonia or amines, which do not contain hydroxide but react with water to increase hydroxide ion concentration. The Lewis theory, developed by G. N. Lewis, defines a base as an electron pair donor; it can share a pair of electrons with an electron acceptor (a Lewis acid), which need not be a proton—boron trifluoride is a notable example. Bases neutralize acids, producing water and a salt in aqueous solution. Strong bases are caustic and react violently with acids; weak bases like baking soda are safer for neutralizing acid spills. Non-hydroxide compounds such as sodium carbonate and ammonia also act as bases by accepting protons in water.
- lewis_theory_proposed_by
- G. N. Lewis
- common_properties
- slippery to touch, bitter taste, pH > 7, turn red litmus blue
Lore & Background
Arrhenius bases are metal hydroxides such as sodium hydroxide (NaOH) or calcium hydroxide (Ca(OH)₂). In aqueous solution, these substances dissociate to release hydroxide ions (OH⁻), which can then react with hydrogen ions from acids to form water. Such solutions are slippery to the touch, taste bitter, and change the color of pH indicators—turning red litmus paper blue, for example. A soluble base that quantitatively releases hydroxide ions is called an alkali. Metal oxides, hydroxides, and alkoxides are basic, and the conjugate bases of weak acids are themselves weak bases. Bases and acids are chemical opposites: acids increase the hydronium ion concentration in water, while bases reduce it. When an aqueous acid and base react, neutralization occurs, producing water and a salt whose ions separate in solution. G. N. Lewis later broadened the definition, describing a base as an electron pair donor. This Lewis theory includes substances like ammonia and water, which possess an unshared pair of electrons that can bond with a proton or other electron acceptors (Lewis acids), such as boron trifluoride. Bases also react with water to form a conjugate acid and hydroxide ion, with the equilibrium constant Kb indicating base strength—a lower constant signifies a weaker base. Strong bases are caustic to organic matter and react violently with acidic substances; weak bases like baking soda or egg white are safer for neutralizing acid spills.
Reader's Guide
Bases are fundamental to acid–base chemistry, with three major definitions reflecting increasing generality. The Arrhenius definition applies to aqueous solutions, where bases produce hydroxide ions. The Brønsted–Lowry definition extends to any proton acceptor, including ammonia, which reacts with water to increase hydroxide concentration. The Lewis definition is the most general, encompassing any electron pair donor, such as water or ammonia, that can bond with an electron acceptor. Bases react with acids in neutralization reactions, producing water and a salt. They have characteristic properties: slippery feel, bitter taste, pH above 7, and they change the color of indicators (e.g., turn red litmus blue). Strong bases are caustic and conduct electricity when dissolved. The equilibrium constant Kb measures base strength; weaker bases have lower Kb values. The concept originated from alchemical notions of a 'matrix' and was formalized by Rouelle in the 18th century. Understanding bases is essential for fields ranging from industrial chemistry to biochemistry.
Did You Know?
- Arrhenius bases are a subset of Brønsted bases, as both involve hydroxide ions or proton acceptance.
- Lewis bases can share an electron pair with an acceptor that is not necessarily a proton, such as boron trifluoride.
- A soluble base that releases OH− ions quantitatively is called an alkali.
Frequently Asked Questions
What is a base in chemistry?
A base is a chemical substance whose core behavior is neutralizing acids, and more broadly, accepting protons or donating electron pairs to other molecules.
Who originally proposed the concept of a base?
The idea of a base as a distinct class of substance was first articulated by G.-F. Rouelle in the mid-1700s, and G. N. Lewis later broadened the definition to include electron-pair donation.
What properties identify a substance as a base?
Bases typically feel slick or slippery on the skin, taste bitter, register a pH above 7, and cause red litmus paper to shift to blue.
What are the three major theories that define a base?
The three standard frameworks are the Arrhenius definition (produces hydroxide ions in water), the Brønsted–Lowry definition (accepts a proton), and the Lewis definition (donates an electron pair).
Why is the base concept central to chemistry fundamentals?
Without a clear notion of a base, chemists would lack a unifying way to describe neutralization reactions, buffer systems, and a wide range of biological and industrial processes.
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