The law of conservation of mass states that mass is neither created nor destroyed. In a closed system, mass of reactants is equal to mass of products. The law of conservation of mass is related to the law of conservation of matter.
In this segment, let's discuss what is one of the most fundamental concepts um in chemistry which is the conservation of mass. So late in the 18th century the analytical balance was developed and so that allowed a scientist to measure how much of your reactant was present and how much of your product was present. Before that that wasn't necessarily a possibility and so what they observed is that although there were chemical changes occurring, the total mass in the reaction actually remained constant and after seeing this time and time again, it led them to believe that mass is neither created nor destroyed and that the mass that is present in the beginning it's present in the end and thus it became the law of the conservation of mass. So the mass of your reactant is equal to the mass of your products.
So there was a scientist back in the 18th century, Antoine Lavoisier, and so he did the thermal decomposition of Mercury 2 oxide. Just as a quick aside, Mercury is a transition metal and this two when it's written this way means that it is in oxidation state two plus. Okay so, the thermal decomposition looks like so, if two mercury oxide solid, you add some heat and you get atomic mercury and oxygen. So the original solid that he had was a powdery red solid and then once he added the heat, he got this silver liquid and oxygen gas. And so performing it a few more times under a closed container, was able to measure the mass of the beginning product, the mercury oxide and that is 216, mercury has a molar mass of about 200 and oxygen is 16 and then the mass at the end of mercury and in oxygen the same thing. The mass of the product was 216 grams, proving that the mass of the reactant side is equal to the mass of the product side and that's in fact conservation of mass does occur.
So the colour change in the production of the gas were indicate as if there was in fact a chemical reaction and so again, we were just able to look at how the process evolved to see that there was mass the mass in the beginning was equal to the mass in the end and that's the fundamental concept of the conservation of mass.