Lemons, oranges and vinegar taste sour because they are acids. Usually, all acids have a sour taste because of the concentration of hydrogen ions. Bases like soaps and toothpaste, on the other hand, taste bitter and are also slippery because they contain metal and hydrogen ions. These substances react with the fats and oils in the skin and thereby causes that “slippery” feeling. Salts are a combination of acids and bases – for example, the common salt – sodium chloride, comprises of both an acid and a base.