Describes something related to chemistry or involving chemical processes. Used as a na-adjective.
This substance is chemically stable.
Chemical treatment is necessary.
化学的 (also read かがくてき) means 'chemical', referring to chemistry, while 科学的 means 'scientific', referring to science in general.
Compound of 化学 (かがく, 'chemistry') + 的 (てき, '-like, -ical').