But you already said it didn't. Quoting you, it doesn't change "weight, molecular structure, chemical characteristics". Those things aren't appearance, they are substance. It doesn't get any more substantial than "weight, molecular structure, chemical characteristics".
"If a person does not believe in the existence of things invisible as well as visible..."
I believe in lots of invisible things, because there is evidence of them. But I don't believe that something both does and does not change at the same time. That's just silly.
Excuse me, but you are mistaking the meaning of “substance,” in this context. This is not surprising, because over the years its common usage in English, has come to mean just as you said -— physical characteristics -— and this is, aggravatingly, the opposite of its earlier philosophical meaning, as in Latin “substantia” or in Greek “ousia.”
Forgive me this, it’s just that its context governs its meaning, and in this case the context is not physical chemistry, but metaphysics.
Briefly, we’re talking about eternal and infinite things being more significant, more consequential, more “real” than their outward signs.
I’m not saying all that to be maddeningly pedantic, but trying to get past the everyday middle-school carnel-minded misunderstandings.
All the outward signs will pass away. The Word was made flesh, and it is Himself -— the Word -— we receive in the flesh.
It’s past mybedtime. I hope my thud,-headedness has not made me too obscure. As Einstein said, “Make everything as simple as possible -— but not simpler.”
You are right to say a thing doesn’t change and not change at the same time. Not if you’re talking about the simple law of noncontradiction, the Third Rule of Logic.
Something can’t be, at the same time and in the same manner, both A and non-A.
However there could be change in a different manner. Like a photon bring both a particle and a wave. That does not insult your intelligence or defy logic, it just points to different manners of being.