Conversation so far:
state and explain law of conversation of mass
The law states the mass of the system must remain constant over time, as system's mass cannot change, so quantity can neither be added nor be removed.
when was the law of conservation of mass formally defined
The first to outline the principle of conservation of mass was given by Mikhail Lomonosov in 1756.

Question: why is the law not always true
Self-contained rewrite: why is the law of conservation of mass not always true