Editing Chemistry (section)

===Acidity and basicity===
[[File:Hydrogen-bromide-3D-vdW.svg|thumb|left|upright=0.8|[[Hydrogen bromide]] exists in the gas phase as a diatomic molecule.]]
{{Main|Acid–base reaction}}

A substance can often be classified as an [[acid]] or a [[base (chemistry)|base]]. There are several different theories which explain acid–base behavior. The simplest is [[Arrhenius acid|Arrhenius theory]], which states that an acid is a substance that produces [[hydronium ion]]s when it is dissolved in water, and a base is one that produces [[hydroxide ion]]s when dissolved in water. According to [[Brønsted–Lowry acid–base theory]], acids are substances that donate a positive [[hydrogen]] [[ion]] to another substance in a chemical reaction; by extension, a base is the substance which receives that hydrogen ion.

A third common theory is [[Lewis acids and bases|Lewis acid–base theory]], which is based on the formation of new chemical bonds. Lewis theory explains that an acid is a substance which is capable of accepting a pair of electrons from another substance during the process of bond formation, while a base is a substance which can provide a pair of electrons to form a new bond. There are several other ways in which a substance may be classified as an acid or a base, as is evident in the history of this concept.<ref>{{cite web |url=https://www.bbc.co.uk/dna/h2g2/A708257 |title=History of Acidity |publisher=BBC |date=27 May 2004 |access-date=12 June 2011 |archive-date=27 February 2009 |archive-url=https://web.archive.org/web/20090227024430/http://www.bbc.co.uk/dna/h2g2/A708257 |url-status=live }}</ref>

Acid strength is commonly measured by two methods. One measurement, based on the Arrhenius definition of acidity, is [[pH]], which is a measurement of the hydronium ion concentration in a solution, as expressed on a negative [[logarithm]]ic scale. Thus, solutions that have a low pH have a high hydronium ion concentration and can be said to be more acidic. The other measurement, based on the Brønsted–Lowry definition, is the [[acid dissociation constant]] (K<sub>a</sub>), which measures the relative ability of a substance to act as an acid under the Brønsted–Lowry definition of an acid. That is, substances with a higher K<sub>a</sub> are more likely to donate hydrogen ions in chemical reactions than those with lower K<sub>a</sub> values.