Change
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.

Dried Semen
Question :

If dried semen is found after having done ghusl of janabah, can it be regarded as tahir, as it has undergone a change, Istihala?

Answer :

The drying of semen is not considered a change into another substance (Istihala), and it will be regarded as najis.