Aspirin is acetylsalicylic acid. As the compound ages, small amounts of water in the air allows some the compound to hydrolyze. This give salicylic acid and acetic acid. Acetic acid is the main ingredient in vinegar, therefor the smell. Solution Aspirin is acetylsalicylic acid. As the compound ages, small amounts of water in the air allows some the compound to hydrolyze. This give salicylic acid and acetic acid. Acetic acid is the main ingredient in vinegar, therefor the smell..