Define the Law of conservation of Mass?

Question

I need help..
Thanks!

Answer 1

ummm...Matter can be neither created nor destroyed.

Answer 2

It's what it says; mass is neither created nor destroyed, so the amount of mass remains constant. This has been known for a long time.

About 100 years ago, Einstein discovered that energy has mass too,

e = mc^2

and so if very large amounts of energy are involved (as in nuclear processes) the law only works if we take into account the mass due to this energy.