Question

Why are chemical changes considered “unseen”?

Answer 1

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A chemical changes is usually considered as  "unseen" because in this type of change structure of a thing or molecule or atom change and because these changes occur at atomic level Hence changes made within a element are so small that it is not possible to seen them just like the physical change which can be easily seen  with help of eye.

Answer 2

Chemical changes occur at the atomic level. They take place within the atoms or molecules. So they are not visible. They change the basic structure of a thing. Therefore they are considered to be 'unseen'. While physical changes appear at the physical level and are visible. For example if a solid changes into a liquid it can be seen.