Which law of thermodynamics states that energy cannot be created or destroyed?

The First Law of Thermodynamics is fundamental in understanding the principle of energy conservation. It asserts that energy in a closed system remains constant; it can only be transformed from one form to another but cannot be created from nothing or destroyed. This means that the total energy within a closed system is conserved, allowing for transformations such as converting chemical energy from food into kinetic energy for movement or thermal energy for heat but never generating energy ex nihilo.

In practical terms, this law underlies many processes in chemistry and physics, emphasizing that all energy exchanges must account for the initial and final states of a system. Understanding this law allows scientists to predict the behavior of systems under various conditions, ensuring that energy input into a reaction or process must match the energy output, plus any losses due to inefficiencies.

The other laws mentioned, such as the Second and Third Laws of Thermodynamics, deal with the directionality of energy processes and the behavior of systems as they approach absolute zero, respectively, without addressing the fundamental idea of energy conservation. The Law of Conservation of Mass, while related to energy in terms of conservation principles, specifically pertains to the mass of substances in a chemical reaction rather than energy.