There are several definitions of what constitutes an acid:

The Arrhenius definition: By the 1884 definition of Svante Arrhenius (Sweden), an acid is a material that can release a proton or hydrogen ion (H+).

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The Lewis definition: A Lewis acid is one that can accept a pair of electrons and form a coordinate covalent bond.

The Brønsted-Lowry definition: Brønsted argued that all acid-base reactions involve the transfer of an H+ ion, or proton. Water reacts with itself, for example, by transferring an H+ ion from one molecule to another to form an H3O+ ion and an OH- ion. According to this theory, an acid is a "proton donor" and a base is a "proton acceptor."

For now we will stick with the Brønsted-Lowry definition. And we will now tie the concept of acids and bases into equilibrium:

Let"s start by looking at the Ionization of Water:

2H2O(l) H3O+(aq) + OH-(aq)

This reaction does fit the Brønsted-Lowry definition since one water molecule is acting as an acid "donating" a proton and the other is acting as a base "accepting" the proton:

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If we now write this reaction into an equilibrium expression:

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But remember one of our rules regarding the writing of equilibrium expressions states that we do not include solvents (liquids) as their concentration stays pretty constant during a reaction so the water portion of the equation is essentially equal to 1 and thus "disappears".

The resulting equation is given a special designation KW called the ionization constant of water:

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The value of this constant at 25oC is 1.0 x 10-14. As you can see from the very small value of KW, water is not very dissociated at room temperature and our assumption that the concentration of water being constant is a good one.

For pure water, the values of <H3O+> and <OH-> are equal and thus their values are both 1.0 x 10-7. If there is an equal amount of acid and base present in any solution, the solution is called "neutral" and the pH of the solution is 7.

If the acid concentration is greater than the base concentration, the solution is acidic and the pH will be less than 7. If the base concentration is greater than the acid concentration, the solution is basic and the pH will be greater than 7.

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The pH Scale:

The pH of a solution is defined as the negative log10

(NOTE: H+ and H3O+ are interchangeable ways to describe the presence of protons in solution)

The pOH of a solution is defined as the negative log10

Conversely,

= 10-pH and = 10-pOH

The pH of a neutral solution is therefore 7. (-log<1 x 10-7> = 7)

The sum of the pH and pOH must always equal 14. This is because the –log KW = -log(1 x 10-14) = 14.