With law of conservation, does mass have to be exactly the same before and after a reaction?

Hal Rouse: The law of conservation of mass is ALWAYS obeyed, no exceptions.If you observe some sort of gain in total mass, something has happened besides the reaction you thought you were performing. (For example, if you were burning Mg in air you might think the product was MgO, but in fact the product would be a mixture of MgO and Mg3N2 and that mixture would weigh more than what you would expect if it were all MgO. In this case the apparent failure of the law was actually a failure to take notice of all the reactants and products.)It's also possible one of your weighings was incorrect. And there's the question of: Can your lab equipment really measure a difference of .017?...Show more