Acids

Acids are one of the largest classes of chemicals whose aqueous solutions have one or more of the following properties: (a) sour taste, (b) ability to turn litmus paper red and cause other indicator dyes to change to characteristic colours, (c) ability to react with and dissolve some metals to form salts, and (d) ability to react with bases or alkali forming salts. All acids contain hydrogen. In water, acids ionize and form hydronium ions (H₃O⁺), usually written simply as hydrogen ions.
Acids are classified as strong acids or weak acids according to the concentration of hydrogen ions that results from ionization. Hydrochloric acid, sulphuric acid, nitric acid and perchloric acid are strong acids since they ionize almost completely in dilute aqueous solutions. Acetic acid and carbonic acid are weak acids. The pH range of acids is from 6.9 to 1 and is a measure of the acid character in an aqueous solution.
In solvents other than water, acid is defined as a substance that ionizes to release the positive ion of the solvent. The Lowry-Bronsted definition of an acid as a substance that can give up a proton is useful in the understanding of a base. The most significant contribution to the theory of acids was the electron-pair concept enunciated by G. N. Lewis. According to Lewis, any molecule or ion that can combine with another molecule by forming a covalent bond with two electrons from the second molecule or ion is an acid. Thus, acid is an electron acceptor like BF₃ or AlCl₃. The hydrogen ion is the simplest substance that will do this.
The terms 'hard' and 'soft' acids and bases refer to the ease with which their electron orbitals can be distorted. Hard acids have positive oxidation state, and their valence electrons are not readily excited. Soft acids have little or no positive charge and possess easily excited valence electrons. Hard acids combine with hard bases and soft acids with soft bases. Soft acids tend to accept electrons and form covalent bonds more readily than hard acids. For example, halogen acids show a progression from hard (HF) to soft (HI).
Major groups of acids are as follows:
(i)Inorganic mineral acids like hydrochloric acid and phosphoric acid.
(ii)Organic acids, whose four major groups are (a) mono carboxylic acids which contain one COOH group (examples include acetic acid), (b) dicarboxylic acid, which contain two COOH groups (examples include pthalic, sebacic and adipic acids), (c) fatty acids which contain long chain COOH groups (examples include oleic, palmitic and stearic acids), and (d) amino acids which contain NIE, and COOH groups (examples include glycine.) Organic acids can also be classified into (a) aliphatic acids, such as acetic acid and formic acid, and (b) aromatic acids, such as benzoic acid and salicylic acid.