Editing Acid (section)

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{{About|acids in chemistry}}
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{{Short description|Chemical compound giving a proton or accepting an electron pair}}

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[[File:Zn reaction with HCl.JPG|thumb|[[Zinc]], a typical metal, reacting with [[hydrochloric acid]], a typical acid]]
{{Acids and bases}}
An '''acid''' is a [[molecule]] or [[ion]] capable of either donating a [[proton]] (i.e. [[Hydron|hydrogen cation]], H<sup>+</sup>), known as a [[Brønsted–Lowry acid–base theory|Brønsted–Lowry acid]], or forming a [[covalent bond]] with an [[electron pair]], known as a [[Lewis acid]].<ref name="IUPAC_acid">[http://goldbook.iupac.org/A00071.html IUPAC Gold Book - acid]</ref>

The first category of acids are the proton donors, or [[Brønsted–Lowry acid–base theory|Brønsted–Lowry acid]]s. In the special case of [[aqueous solution]]s, proton donors form the [[hydronium ion]] H<sub>3</sub>O<sup>+</sup> and are known as [[Acid–base reaction#Arrhenius theory|Arrhenius acids]]. [[Johannes Nicolaus Brønsted|Brønsted]] and [[Martin Lowry|Lowry]] generalized the Arrhenius theory to include non-aqueous [[solvent]]s. A Brønsted–Lowry or Arrhenius acid usually contains a hydrogen atom bonded to a chemical structure that is still energetically favorable after loss of H<sup>+</sup>.

Aqueous Arrhenius acids have characteristic properties that provide a practical description of an acid.<ref>{{cite book |last1=Petrucci |first1=R. H. |last2=Harwood |first2=R. S. |last3=Herring |first3=F. G. |title=General Chemistry: Principles and Modern Applications |date=2002 |publisher=Prentice Hall |isbn=0-13-014329-4 |page=146 |edition=8th}}</ref> Acids form aqueous solutions with a sour taste, can turn blue [[litmus]] red, and react with [[Base (chemistry)|bases]] and certain metals (like [[calcium]]) to form [[Salt (chemistry)|salts]]. The word ''acid'' is derived from the [[Latin]] {{lang|la|acidus}}, meaning 'sour'.<ref>[http://www.merriam-webster.com/dictionary/acid Merriam-Webster's Online Dictionary: ''acid'']</ref> An aqueous solution of an acid has a [[pH]] less than 7 and is colloquially also referred to as "acid" (as in "dissolved in acid"), while the strict definition refers only to the [[solute]].<ref name="IUPAC_acid"/> A lower pH means a higher '''acidity''', and thus a higher concentration of hydrogen cations in the solution. Chemicals or substances having the property of an acid are said to be '''acidic'''.

Common aqueous acids include [[hydrochloric acid]] (a solution of [[hydrogen chloride]] that is found in [[gastric acid]] in the stomach and activates [[digestive enzymes]]), [[acetic acid]] (vinegar is a dilute aqueous solution of this liquid), [[sulfuric acid]] (used in [[car battery|car batteries]]), and [[citric acid]] (found in citrus fruits). As these examples show, acids (in the colloquial sense) can be solutions or pure substances, and can be derived from acids (in the strict<ref name="IUPAC_acid"/> sense) that are solids, liquids, or gases. [[Acid strength|Strong acid]]s and some concentrated weak acids are [[corrosive substance|corrosive]], but there are exceptions such as [[carborane]]s and [[boric acid]].

The second category of acids are [[Lewis acids and bases|Lewis acids]], which form a covalent bond with an electron pair. An example is [[boron trifluoride]] (BF<sub>3</sub>), whose boron atom has a vacant [[atomic orbital|orbital]] that can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in [[ammonia]] (NH<sub>3</sub>). [[Gilbert N. Lewis|Lewis]] considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly ''or'' by releasing protons (H<sup>+</sup>) into the solution, which then accept electron pairs. Hydrogen chloride, acetic acid, and most other Brønsted–Lowry acids cannot form a covalent bond with an electron pair, however, and are therefore not Lewis acids.<ref name="Oxtoby8th">{{cite book |last1=Otoxby |first1=D. W. |last2=Gillis |first2=H. P. |last3=Butler |first3=L. J. |title=Principles of Modern Chemistry |date=2015 |publisher=Brooks Cole |isbn=978-1305079113 |page=617 |edition=8th}}</ref> Conversely, many Lewis acids are not Arrhenius or Brønsted–Lowry acids. In modern terminology, an ''acid'' is implicitly a Brønsted acid and not a Lewis acid, since chemists almost always refer to a Lewis acid explicitly as such.<ref name="Oxtoby8th" />