<span>The correct answer here is that the reason chemical changes can be considered unseen, is because the alterations made within an element when it undergoes a chemical change occurs within the very atoms of the cell itself. In contrast with this, a physical change (such as a solid changing to a liquid through melting) does not change the element itself, but just it's physical form, so this can be seen easily to the human eye.</span>
Answer is: chemical changes are considered "unseen", because they occur at the atomic level.
There are two types of substance changes: chemical and physical.
In chemical change (chemical reaction) new substances are formed, the atoms are rearranged (atoms are invisible to human eyes).
Physical property can be observed and measured without any changes in molecular composition. The same substance is present before and after the psysical change, just with different form or state of matter, this changes are seen by human eyes because of change of color or state of matter.
