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# Acid

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An acid (from Latin acidus, meaning sour or tart) is a chemical compound generally defined by its reactions with complementary chemicals, designated bases (see Acid-base reaction theories). An acid tends to give a proton and can be represented by the generic formula AH. In water, there is the following reaction:

$\mbox{AH} +\mbox{H}_2\mbox{O} \Leftrightarrow \mbox{A}^- + \mbox{H}_3\mbox{O}^+$

There is a distinction between weak acids and strong acids. For a strong acid, no AH remains in solution:

$\mbox{AH} +\mbox{H}_2\mbox{O} \rightarrow \mbox{A}^- + \mbox{H}_3\mbox{O}^+$

That is why the acidity constant is only defined for weak acids:

Ka = [A-][H3O+]/[AH].

Some of the stronger acids include the hydrohalic acids - HCl, HBr, and HI - and the oxyacids, which tend to contain central atoms in high oxidation states surrounded by oxygen - including HNO3, H2SO4, HClO4.

Acidity is typically measured using the pH scale.

Acidic (chemistry), the opposite to basic, reacting with basics to form salts. Acidic (geology), of rock: containing more than 65% of silica.

Characteristics Acids are generally:

• Taste: sour when dissolved in water
• Touch: strong acids have a stinging feeling
• Reactivity: acids react violently with many metals
• Electrical Conductivity: acids are electrolytes

Different Definitions of Acid/Base The word acid comes from the Latin acidus meaning sour. Chemically though the term acid has a more specific meaning.

The Swedish chemist Svante Arrhenius defined an acid to be a substance that gave up hydrogen ions (H+) when dissolved in water, while bases are substances that give up hydroxide ions (OH-). Notice that this definition limits acids and bases to substances that can dissolve in water. Later on, Bronsted[?] and Lowry[?] defined an acid to be a proton donor and a base to be a proton acceptor. In this definition, even substances that are insoluble in water can be acids and bases. The most general definition of acids and bases is the Lewis definition. A Lewis acid is an electron acceptor, while a Lewis base[?] is an electron donor. Acid/base systems are different from redox reactions in that there is no change in oxidation state.

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