Question

How can you be certain that a chemical change has occurred?

a) there is at least one substance that was not present before the change
b) the amount of measurable material is different after the change
c) the change substance have become warmer
d) at least one substance undergoes a change of state

Answer 1

I'm going to say that the answer is a, because if a chemical change takes place, the substance is going to be different.
Hope this helps