use recrystallisation as the impurities stay in solution when the solution is acidified. the aspirin or acetyl-salicylic acid is insoluble in acidic conditions. you then filter off and rinse with acid. you then dissolve and filter again in case any insoluble impurities got caught up in the crystal. you then acidify again to recrystallise and filter off liquid. this will leave pure aspirin. don't forget to dry it!.
doing it with evaporation will leave all the impurities in the crystals and you may not even get the desired product!