Almashinish, ajralish, birikish, o'rin olish, Bitta murakkab moddadan ikkita yoki undan ortiq modda hosil bo’lsa, bunday reaksiyalar ajralish reaksiyasi deyiladi, Ikki yoki undan ortiq oddiy yoki murakkab moddalar (molekulalarining) birikishidan bitta murakkab modda hosil bo’lsa, bunday reaksiya birikish reaksiyasi deyiladi