Yeast alone is not chametz (nowadays they put Chametz additives into it)
?????
Yeast is a microorganism that flies in the air. When it lands on something with sugars, it ferments it. Sometimes, instead of waiting for yeast to land on our food, we take a bit of food that already has yeast in it and stick that in. Those bits of food are what we call “yeast” that we buy in the store.
If those bits of food are dough for instance (as should be the case if it is bread yeast), then I do think it is chometz.