Question :

Does Istihala occur when any component undergoes a chemical reaction?

Answer :

We cannot present a general principle for this, since the change that results in the taharah of the changed substance occurs when the second state is different from the first one in the conventional understanding, in such a way that people – within conventional norms – consider the second form as a different matter.
An obvious example is the Istihala of wood into smoke or ash.