Tooth enamel is made of calcium phosphate. It does not dissolve in water but it starts corroding when the pH in the mouth decreases to 5.5 or less. When we eat chocolates, small pieces of these stick to our teeth. The bacteria present in our teeth start decaying the food particles as a result of which small amount of acid is produced. This acid, decreases the pH of the mouth and results in the decaying of tooth enamel.