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.
why is the law of conservation of mass not always true
Mass is also not generally conserved in open systems. Such is the case when various forms of energy and matter are allowed into, or out of, the system.

Question: who discovered the law
Self-contained rewrite: who discovered the law of conservation of mass