Which of the following states that energy cannot be created or destroyed in an isolated system?

The statement that energy cannot be created or destroyed in an isolated system is described by the first law of thermodynamics. This principle establishes the concept of conservation of energy, which asserts that the total energy of an isolated system remains constant, although it can change forms (for example, from kinetic to potential energy or thermal energy).

In practical terms, this means that when energy is transferred or transformed—such as in mechanical work or heat transfer—the total energy before and after the transformation remains the same. This law is foundational in understanding various phenomena in physics and chemistry, especially in processes like chemical reactions and phase changes.

The other options relate to different principles; the second law of thermodynamics involves the directionality of energy transfers and the concept of entropy, Newton's laws pertain to the motion of objects, and the law of conservation of mass focuses on the mass of substances undergoing chemical reactions. None of these succinctly capture the essence of energy conservation as stated in the first law of thermodynamics.