Chemistry Reference

What is an Acid and a Base?
There are three different definitions for an acid. The one that is most frequently used in introductory chemistry courses is named after the Swedish chemist Svante Arrhenius. Arrhenius acids are compounds that dissociate in water and yield hydrogen ions (H+). Common compounds that fit this definition include hydrochloric acid and sulfuric acid. In fact, it is safe to say that inorganic compounds that have a formula that begins with "H" are Arrhenius acids. Conversely, bases are compounds that dissociate in water and yield hydroxide (OH-) ions. Examples of Arrhenius bases are sodium hydroxide and calcium hydroxide.
Organic Acids
Recall that organic compounds are those that feature carbon, with emphasis on carbon-carbon and carbon-hydrogen bonds. Some common organic acids are acetic acid and citric acid. Organic acids are commonly referred to as carboxylic acids, and they feature the -COOH functional group. In many formulas, the -COOH appears at the end of the formula. The aforementioned acetic acid if often written as CH3COOH. It should be noted that although these compounds also end in -OH like bases, the -COOH group is a different functional group. The -OH group is attached to metals and polyatomic ions, and do not contain the C-C and C-H bonds that would make it organic. Therefore, compounds that contain only -OH are not organic acids.
Reactions
The Arrhenius definition of acids and bases provides the basis for a simple double displacement reaction. Consider the reaction shown below:
In this reaction, the Arrhenius acid is H3PO4, or phosphoric acid. The presence of H at the beginning of the formula is a clue that it is the acid. Similarly, the second compound, NaOH (sodium hydroxide), must be the base because it features a metal (Na) bonded to the hydroxide (OH-) ion. In the reaction between an Arrhenius acid and base, water will always be produced. This is because the reaction is double displacement and the arrangement of the acid's cation (always H+) with the base's anion (always OH-) yields the compound HOH, or H2O. The second product is referred to as the salt, and is made of acid's anion and the base's cation. It is often, but not always aqueous. Recall that double displacement reactions only work when one of the products are not aqueous. This is true of the product water, which is always a liquid.
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