An acid is any substance whose aqueous solution is characterized by a sour taste, the ability to turn blue litmus red, and the ability to react with bases and certain metals to form salts. A Base is any substance whose aqueous solution is characterized by a bitter taste, a slippery feel, the ability to turn red litmus blue, and the ability to react with acids to form salts. Questions about the acidity and alkalinity of solutions and different types of acids and bases are found n our Acids and Bases category.