Ghost Gas

The balance reading keeps changing on its own. Heavier here, lighter there. Is the lab haunted? No: it's gas. Every gas particle has mass, but you can't see them, and the balance only weighs what is sitting on it. Read each balanced symbol equation, call whether the mass will go up, down or stay the same, then scan the gas molecules as they join a hot metal or escape from an open container. Then seal a flask and prove that mass is conserved.