Chametz is Unique
There is another unique aspect to the prohibition of eating Chametz on Pesach. The penalty for such eating is “Karet”, which is one of the most severe punishments in the Torah. It is equal to the prohibition of eating on Yom Kippur.
There is a general principle related to Kashrut laws of בטל בששים, that a prohibited item is nullified if it became mixed with sixty times more of a permitted idea. The taste of the forbidden item is no longer felt.
This principle also applies to Chametz until Pesach begins. This is why there are many items that say, “Kosher for Pesach,” if purchased before the holiday. If there was a tiny amount of Chametz, it would be nullified by the sixty to one principle.
However, once Pesach begins, the Rabbis instituted the concept of חמץ אסור במשהו, that Chametz is forbidden with the slightest amount. This is the source of why we are so careful with our cleaning and our purchases.
There are two reasons given for this stringency. One is referred to as לא בדיליה מיניה, that people do not know how to separate from something they are used to using.
The second reason is that Chametz is referred to as a דבר שיש לו מתירין, something that will eventually become permitted. This refers to after the Chag, and items defined in this category are never nullified.
This gives us an idea of why Chametz is so unique, and the great care needed to be taken, to avoid its prohibition.