Acid

Name:
Acid
Detailed information:
One of a large class of chemical substances whose water solutions have one or more of the following properties: sour taste, ability to make litmus dye turn red and to cause other indicator dyes to change to characteristic colors, ability to react with and dissolve certain metals to form salts, and ability to react with bases or alkalies to form salts. All acids contain hydrogen. In water, ionization or splitting of the molecule occurs so that some or most of this hydrogen forms H₃O⁺ ions (hydronium ions), usually written more simply as H⁺ (hydrogen ion). Acids are referred to as strong or weak according to the concentration of H⁺ ion that results from ionization. Hydrochloric, nitric, and sulfuric are strong or highly ionized acids; acetic acid (CH₃COOH) and carbonic acid (H₂CO₃) are weak acids. Tenth normal hydrochloric acid is 100 times as acid (pH = 1) as tenth normal acetic acid (pH = 3). The pH range of acids is from 6.9 to 1.See pH.When dealing with chemical reactions in solvents other than water, it is sometimes convenient to define an acid as a substance that ionizes to give the positive ion of the solvent. The common definitions of acid have been extended as more detailed studies of chemical reactions have been made. The Lowry-Brønsted definition of an acid as a substance that can give up a proton is more useful in connection with an understanding of bases (see Base). Perhaps the most significant contribution to the theory of acids was the electron-pair concept introduced by G. N. Lewis around 1915.See Lewis Electron Theory.The terms hard and soft acids and bases refer to the ease with which the electron orbitals can be disturbed or distorted. Hard acids have a high positive oxidation state, and their valence electrons are not readily excited; soft acids and bases have little or no positive charge and easily excited valence electrons. Hard acids combine preferentially 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, the halogen acids arranged in a series by increasing atomic weight (and decreasing chemical activity) show a progression from hard (HF) to soft (HI). A brief outline of the major groups of acids is as follows:InorganicMineral acids: sulfuric, nitric, hydrochloric, phosphoric.Hazard: All mineral acids are highly irritant and corrosive to human tissue.OrganicCarboxylic (contain —COOH group)aliphatic: acetic, formicaromatic: benzoic, salicylicDicarboxylic (contain two —COOH groups)oxalic, phthalic, sebacic, adipicFatty acids (contain —COOH group)aliphatic: oleic, palmitic, stearicaromatic: phenylstearicAmino acids: N-containing protein componentsSee Lewis Acid; Carboxylic Acid; Fatty Acid; Amino Acid; specific compounds.