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.

The answer is a, (There must be a change in chemical properties) because in a chemical reaction, new substances are formed.
It's not c since the rule of conservation of mass states that the mass does not decrease or increase since no particles are made or lost. The mass of the products is equal to the mass of the reactants. It's not b or d since there's not ALWAYS a change in physical properties or volume.

Hello there.

What must occur for a change to be a chemical reaction?

a.There must be a change in chemical properties.