How can the law of conservation of matter be demonstrated?
one way is through annihilation of ordinary matter with antimatter
Einstein showed energy and mass are in fact the same thing and that the law of conservation of energy is same as the law of conservation of mass.
therefore, mass can disappear suddenly but only if an equal amount of energy is produced which is demonstrated by Einstein's famous equation
E = energy,
m = mass,
c = speed of light in vacuum
and if an electron and a positron (antimatter of electron) come close to each other, due to their unlike charges, they attract and disappear after contacting through a process known as annihilation , and release energy equivalent to their masses and respective energies, in the form of gamma radiation photons
therefore, even when mass disappears, it takes the form of energy and is not just "lost"
this also proves that the law of conservation of matter and energy are the same