Question

what is the outcome of all chemical changes when two substances are combined?
a new substance forms with identical properties.
the two substances release an odor.
the two substances change color.
a new substance forms with different properties.

Explanation:

A chemical change involves a transformation at the molecular level. When two substances combine chemically, bonds are broken and new bonds are formed. This results in the creation of a new substance. By definition, this new substance has different chemical and physical properties compared to the original substances. For example, when hydrogen ($\ce{H2}$) and oxygen ($\ce{O2}$) combine chemically to form water ($\ce{H2O}$), hydrogen is flammable, oxygen supports combustion, but water is used to extinguish fires - showing different properties. Releasing an odor or changing color (while they can be signs of a chemical change) are not the universal outcome of all chemical changes. A new substance with identical properties would imply no chemical change occurred as the identity (properties) of substances is key in chemical reactions.

Answer:

A new substance forms with different properties.