An acid is a substance that donates protons (in the Bronsted-Lowry definition) or accepts a pair of valence electrons to form a bond (in the Lewis definition). acid, any substance that in water solution tastes sour, changes the color of certain indicators (e.g., reddens blue litmus paper), reacts with some metals (e.g., iron) to liberate hydrogen, reacts with bases to form salts, They react with the calcium carbonate to give a gas and promotes certain chemical reactions (acid catalysis).