How does the Law of conservation relate to the reactants and products in a chemical reaction?

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction (in a nuclear reaction it is a different matter). Therefore the total mass of the reactants equals the total mass of the products. This law is hard to grasp by some since some reactions are gas creating reactions, and most reactions occur in open systems; Therefore, the gas escapes and cannot be weighed properly, but mass is still conserved.