In the reaction between salicylic acid and acetic anhydride to produce aspirin, why is it necessary to quench the reaction?
Acetic anhydride is contaminating your final product. Quench simply means to destroy excess reagent before continuing. It might not matter too much in this particularly reaction since you're about to purify via recrystallization anyway, but imagine if the next step involved adding a very expensive nucleophile. That nucleophile would add to acetic anhydride and be wasted.