Thermodynamic stability is a term used in chemistry to describe a chemical system that is neither consuming nor releasing heat energy. In the absence of a change in thermal energy, the substance is not undergoing a chemical reaction and is, therefore, stable. In mixtures that are thermodynamically stable, the stable state occurs either before any of the chemical reactants have been transformed into chemical products or after that process is complete. If a chemical system is thermodynamically stable, there is no movement of heat within a system or between a system and the surrounding environment.
Scientists can measure the amount of heat energy that produced by or put into a chemical reaction. Some chemical systems will automatically react with one another when they come into contact. Thermodynamic reactions that occur spontaneously and give off heat are known as exothermic reactions. Though it is possible to add heat energy to this type of system, an act that usually functions to speed up the reaction, there is enough energy within the reactants themselves to transform the reactants into products. Once the reaction is complete, the chemical product, which is the same chemical system that the reactants once made up, is said to be in a state of thermodynamic stability.
Reactants may need to consume energy in order to be transformed into products. In these types of chemical systems, thermodynamic stability occurs before energy is added to the system. In the absence of extra heat energy, the chemicals in the system will not react with one another. They are able to resist being transformed into products. This type of thermodynamic stability occurs in endothermic reactions.
An example of an exothermic chemical reaction that results in a thermodynamically stable product is the transformation of sugar and water into sugar-water. When sugar is added to water, the reactants form a thermodynamically unstable system. The sugar begins to automatically dissolve into the water, a process that continues until all of the sugar is gone. At this point, the system is thermodynamically stable because it would take the addition of energy into the system to reverse the chemical process and separate the water and the sugar from one another. True states of thermodynamic stability are rare because chemical systems are almost always in the process of reacting in some way.