In chemistry, there are two laws: the Conservation of Matter, and the Conservation of Energy. They each state that matter/energy can be neither created nor destroyed *by normal chemical means*.
However, each can be destroyed, in a sense, in physics.
An example is the atom bomb. By nuclear fission, the nucleus of an isotope of the element uranium, uranium 235, is blown apart. In this process, uranium 235 loses matter, but this lost, or "destroyed", matter is converted into an equivalent amount of energy, thus creating the highly destructive mushroom cloud of radioactive energy.
Einstein recognized that the laws of Conservation of Matter and Energy apply to chemistry and other sciences, but during his work with nuclear physics, he modified the law to one almost everyone is familiar with: E = mc^2.
Where E = energy, m = matter, and c = the speed of light, Einstein's equation states that where matter is lost or destroyed, an equivalent amount of energy is created, and vice versa. Where energy appears to be lost, an equal amount of matter is created.
Thus the laws of Conservation of Matter and Energy are applicable, unless dealing with the highly theoretical science of nuclear physics.
