Based on the law of Conservation of Mass, what happens to the mass of a substance in a chemical change? A: the mass is always conserved B: The mass is either created or destroyed C: the reactant always has more mass D: the product always has more mass

A.

Explanation:

The amount of mass always stays the same, matter cannot be created nor destroyed, it can only change forms. Thus, the answer is A.  

pure substance: iron,copper

impure substance: water,charcoal

the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.