A new mishnah! A handful of one grain-offering that is mixed with another handful of grain... according to the rabbis' view, the mingling does not invalidate the sacrifice. Burning it on the altar is fine. But Rabbi Yehudah notes that select grain-offerings would not be valid in these circumstances. The Gemara then delves into parsing Rabbi Yehudah's position here, with comparison to the animal sacrifices of Zevahim. Plus, the question of when and whether substances can be nullified in the mixture of something of its same kind or something that much more different from the original. Note - these laws of mixtures are relevant to all areas of Halakhah, not just the sacrificial realm... except that the "no nullification" rule IS specific to the sacrificial realm. Or at least, when it's items of the same substance and also being offered on the altar. But the case is determined to be difficult.