Acid

An acid "is a chemical compound that disassociates in a solution, releasing hydrogen ions and lowering the pH (it is a proton donor). An acidic solution has a pH below 7.0." They have a sour taste (but don't go around tasting random acids!),  are capable of turning a litmus test red, and can form salts when they react with bases, metals, and alkalis. The word acid comes from the Latin word "acere," meaning sour.

The sour taste found in limes and other citrus fruits  is a result of all the acid  inside.