Which statement describes the law of constant composition (definite proportions)? mass in an isolated system is neither created nor destroyed by chemical reactions. all samples of a given chemical compound will be composed of the same elements in the same proportion. when two elements react to form multiple compounds, the different masses of one element that combine with a fixed mass of the other element can be expressed as a ratio of whole numbers. energy cannot be created or destroyed, but may be changed from one form to another. for a collision occurring between two particles in an isolated system, the total momentum of the two particles before the collision is equal to the total momentum of the two particles after the collision.