![]() ![]() The pH of a basic solution at standard conditions is greater than seven.Reactions with indicators: bases turn red litmus paper blue, phenolphthalein pink, keep bromothymol blue in its natural colour of blue, and turn methyl orange-yellow.Aqueous solutions or molten bases dissociate in ions and conduct electricity.Concentrated or strong bases are caustic on organic matter and react violently with acidic substances.Some other definitions of both bases and acids have been proposed in the past, but are not commonly used today. One notable example is boron trifluoride (BF 3). The Lewis theory is more general than the Brønsted model because the Lewis acid is not necessarily a proton, but can be another molecule (or ion) with a vacant low-lying orbital which can accept a pair of electrons. In the Lewis theory, a base is an electron pair donor which can share a pair of electrons with an electron acceptor which is described as a Lewis acid. Lewis realized that water, ammonia, and other bases can form a bond with a proton due to the unshared pair of electrons that the bases possess. For example in liquid ammonia, NH 2 − is the basic ion species which accepts protons from NH 4 +, the acidic species in this solvent. Also, some non-aqueous solvents contain Brønsted bases which react with solvated protons. These bases do not contain a hydroxide ion but nevertheless react with water, resulting in an increase in the concentration of hydroxide ion. However, there are also other Brønsted bases which accept protons, such as aqueous solutions of ammonia (NH 3) or its organic derivatives ( amines). This does include aqueous hydroxides since OH − does react with H + to form water, so that Arrhenius bases are a subset of Brønsted bases. In the more general Brønsted–Lowry acid–base theory (1923), a base is a substance that can accept hydrogen cations (H +)-otherwise known as protons. If the aqueous solution is saturated with a given salt solute, any additional such salt precipitates out of the solution. A reaction between aqueous solutions of an acid and a base is called neutralization, producing a solution of water and a salt in which the salt separates into its component ions. Metal oxides, hydroxides, and especially alkoxides are basic, and conjugate bases of weak acids are weak bases.īases and acids are seen as chemical opposites because the effect of an acid is to increase the hydronium (H 3O +) concentration in water, whereas bases reduce this concentration. A soluble base is called an alkali if it contains and releases OH − ions quantitatively. In water, by altering the autoionization equilibrium, bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the water has a pH higher than 7.0 at standard conditions. They are slippery to the touch, can taste bitter and change the color of pH indicators (e.g., turn red litmus paper blue). Such aqueous hydroxide solutions were also described by certain characteristic properties. ![]() A base was therefore a metal hydroxide such as NaOH or Ca(OH) 2. These ions can react with hydrogen ions (H + according to Arrhenius) from the dissociation of acids to form water in an acid–base reaction. In 1884, Svante Arrhenius proposed that a base is a substance which dissociates in aqueous solution to form hydroxide ions OH −. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Magnesia, a district of Eastern Thessaly in GreeceĪlumina, from Latin alumen (gen.In chemistry, there are three definitions in common use of the word " base": Arrhenius bases, Brønsted bases, and Lewis bases. Symbol Na is derived from Neo-Latin natrium, coined from German Natron, ' natron'.Greek elements hydro- and -gen, ' water-forming'īeryl, a mineral (ultimately from the name of Belur in southern India) īorax, a mineral (from Arabic bawraq, Middle Persian * bōrag)Ĭoined by Humphry Davy who first isolated it, from English soda (specifically caustic soda), via Italian from Arabic ṣudāʕ 'headache' ![]()
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