Question

Nuclear Fission/fusion occurs with destruction of same mass and evolution of large amount of energy.But law of conservation of mass says that mass can neither be created nor be destroyed. How?

Answer 1

Law of Conservation of Matter, proven by Lavoisier in 1785: matter can be neither created nor destroyed.

Matter is never destroyed inside of a nuclear reaction, it is simply transferred to a different state. Matter is made from energy at the tiniest, most quantum levels, and the energy gets transferred from one place to another, or from one state to another. When we see a nuclear rection blow something up or "destroy" something, it is actually trasferring that energy elsewhere into different elements.