System

Its reading can therefore change if matter crosses the boundary of the system being weighed. Gas escaping from an open container is no longer supported by the balance, while gas entering from the air adds matter that was not initially on it. Neither observation contradicts conservation of mass.

Conservation applies to the complete system. It is easiest to observe in a closed system, where no matter can enter or leave. If the container and all its contents are weighed together, their total mass before the reaction equals their total mass afterwards.