How do you write chemical equations?
Stoichiometrically: mass is always conserved.
Chemical equations reflect a real chemical or physical change. It is a fundamental underlying principle of chemistry that mass is conserved. What does this mean? It means that if I start with 10 g of reactant, from all sources, at most I am going to get 10 g of product. In practice I am not even going to get that.
Let's take a simple hydrocarbon combustion reaction, the which drives your motor:
Is mass conserved here? Well, it seems to be. Because for every reactant particle there is a corresponding product particle. If there were not, I would know that the equation was unbalanced and therefore unsound. In practice of course, hexane would not burn so cleanly. Some of the product would be
This principle of conservation may be pushed farther. Not only is mass conserved, but charge and energy is conserved as well. I started with neutral reactants; I finished with neutral products. The energy content of the starting hydrocarbon was also conserved; I could use some of the energy evolved in the reaction above to do quantitfiable work.