The Torah law concerning a ĥametz mixture is complicated and subject to dispute among Tanna’im, Amora’im, Rishonim, and Aĥaronim. We shall summarize its laws here succinctly.
The Torah declares that one who eats a kezayit of ĥametz on Pesaĥ incurs karet. If the ĥametz is mixed with other foods and the mixture contains a kezayit of ĥametz in a shi’ur akhilat pras of kosher food (an olive’s bulk of ĥametz in three or four eggs’ bulk of kosher food), Ramban and other Rishonim rule that his punishment is karet, while Rif and Rambam maintain that his punishment is only malkot (lashes).
If ĥametz mixes with the same type of food, for example, leavened flour with unleavened flour, most poskim maintain that since they taste the same, the ĥametz flour is nullified by the majority (batel be-rov) at the Torah level, though there is still a rabbinic prohibition against eating it.
When it comes to the prohibition against keeping ĥametz on Pesaĥ, if a kezayit of ĥametz becomes mixed with permitted food, and the permitted food is less than sixty times the quantity of the ĥametz, one violates both bal yera’eh and bal yimatzei. According to Torah law, if the mixture is more than sixty times the quantity of the ĥametz, the ĥametz is batel. If the ĥametz becomes mixed with its own kind – for example, leavened flour with unleavened flour – and there is more of the kosher ingredient than the non-kosher, according to Torah law the ĥametz is batel and no prohibition is violated. Nevertheless, the Sages ordained that the mixture must be disposed of, lest one end up eating it on Pesaĥ.