Acids will react with certain metals to liberate hydrogen gas. Acids cause color changes in acid-base indicators. Acids react with hydroxides to form water and a salt. Dilute aqueous solutions of acids have a sour taste. These properties can be observed experimentally, and form the basis of the operational definition of an acid.
Acids conduct electricity in relation to the degree of their ionization.
Acids will react with those metals below hydrogen in the Table of Standard Electrode Potentials given in Reference Tables for Chemistry. Some acids, in addition to their acid properties, have strong oxidizing ability and thus, except in very dilute solution, do not release hydrogen gas on reaction with metals. For example, nitric acid and concentrated sulfuric acid have strong oxidizing properties. Acid-base indicators are substances which have different colors in acid and basic solutions. Two common indicators are litmus which is blue in basic solution and red in acid solution, and phenolphthalein which is pink in basic solution and colorless in acid solution.