what is the law of conversation of matter?

Answer 1

you can't create or destroy matter, but you can change it's state

Answer 2

That changes in energy states show up in different manifestations, like boiling water changing to steam. The point is that nothing was lost, just changed.
This law is probably the most important proof of spirituality and possibly God, because if this law is true, then the thoughts and emotions of a person, at death, are transmuted into something, somewhere, which could be the 'after life." The law can account for the existence of a soul.

Answer 3

Don't you meean conservation, my dear? As to the law of "conversation of matter", there is not much of a law when you do converse about matter because people who deals with them talks about them in a much more understandable and open manners, right?