What must be done to be certain that a chemical change has taken place?
a. check for the production of bubbles before and after the change.
b. demonstrate that a release of energy occurred after the change.
c. check the composition of the sample before and after the change.
d. demonstrate that energy was absorbed by the reactants after the change.

Option (c) is the correct answer.

Explanation:

Chemical changes are defined as the change which tend to show difference in chemical composition of a substance.

For example, toxicity, reactivity, combustion etc are all chemical properties.

A chemical change will always lead to the formation of a new compound and this change is irreversible in nature by any physical means.

Formation of bubbles is not necessary in every chemical reaction. But change in composition of the reactants will always occur in a chemical change.

Thus, we can conclude that check the composition of the sample before and after the change, to be certain that a chemical change has taken place.