What is carbon-12?

The Law of Conservation of Mass:The mass of a closed system will stay constant over time, regardless of the processes occurring inside the system. Mass cannot be made or destroyed but, it can be rearranged and changed into different types of particles. For example, when water is created from hydrogen and oxygen, atoms of each element are not lost or created; just rearranged.
The Law of Constant Composition: A compound always has the same proportion of elements by mass. In H2O, every compound has 2g of hydrogen and 16g of oxygen.