Any substance that in water solution tastes sour, changes the colour of acid-base indicators (e.g., litmus ), reacts with some metal s (e.g., iron) to yield hydrogen gas, reacts with base s to form salt s, and promotes certain chemical reaction s (e.g., acid catalysis ).
Acids contain one or more hydrogen atoms that, in solution, dissociate as positively charged hydrogen ion s. Inorganic, or mineral, acids include sulfuric acid , nitric acid , hydrochloric acid , and phosphoric acid. Organic acids include carboxylic acid s, phenol s, and sulfonic acids. Broader definitions of acids cover situations in which water is not present. See also acid-base theory .