An acid is a chemical substance that neutralizes alkalis, dissolves some metals, and turns litmus red; typically, a corrosive or sour tasting liquid of this kind. Arrhenius: a chemical compound that increases the the concentration of hydrogen ions, H, in aqueous solution. The acid will ionize in solution, increasing the number of hydrogen ions present. Bronsted- Lowry Acids is a molecule or ion that is a proton donor. Because H+ is a proton, all acids as defined by Arrhenius donate protons to water and are Bronsted-Lowry acids