[Avodah] What is Chametz?

[Alan Rubin]

Is chametz a purely Halachic construct, an empirically observable
phenomenon or something else?

Evidence for it not being an empirically observable phenomenon.

Chametz can only occur with 5 species. Is it not possible that other
species might also leaven?

Pure Mei Peyros cannot make chametz. But Mei Peyros is mostly water
and presumably bread made with it looks like it has leavened.

Evidence for it being an empirically observable phenomenon.

Doesn't the gemara in the dispute over whether rice leavens or rots
describe observing it to see what happens. (I might have remembered
this wrong)

So what is chametz?

Alan Rubin