A base is a molecule which produces hydroxide ions when added to water (according to the Arrhenius definition), which accepts H+ ions in a chemical reaction (according to the Bronsted-Lowry definition), or which donates an electron pair in a reaction (according to the Lewis definition). Bases have a bitter taste and turn red litmus paper blue. Bases feel slippery when dissolved in water. Most strong bases contain a hydroxide ion. Bases are indicated by numbers from above 7 to 14 on the pH scale, and are the opposite of acids.