What must occur for a change to be a chemical reaction?
a.
there must be a change in chemical properties.
b.
there must be a change in physical properties.
c.
the change must involve a change in mass.
d.
the change must involve a change in volume.

For a chemical reaction to have occurred, there must be a change in chemical properties of the substances formed in comparison to the substances initially being present prior to the reaction. The starting reactants will have very different chemical properties and reactivities due to the atomic bonding existing than the newly formed products, which will also have different chemical properties due to the arrangement of atoms and the chemical bonding that occurs between the atoms.

For the answer to the question above,  I believe the answer is the first one, that's why they call it a "chemical" reaction.
So the answer is a.
There must be a change in chemical properties.

I hope my answer helped you. Feel free to ask more questions. Have a nice day!

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