Thermal equilibrium is achieved when two objects or systems reach the same temperature and cease to exchange energy through heat. When two objects are placed together, the object with more heat energy will lose that energy to the object with less heat energy. Eventually, their temperatures will be equal and they will cease to exchange heat energy as neither object is warmer or cooler than the other. At this point, they are in a state of thermal equilibrium.
Thermal contact is an important concept relating to thermal equilibrium. Multiple systems are considered to be in thermal contact if they are capable of affecting the others' temperatures. If a bottle of soda is removed from the refrigerator and placed on the kitchen counter, which is at room temperature, the counter and the bottle of soda are in thermal contact. Heat energy from the counter flows to the cold bottle of soda. Eventually, their temperatures will be equal and they will be in a state of thermal equilibrium.
In thermal systems involving objects in thermal contact, heat flows from the warmer object, which contains more thermal energy, to the cooler object, which contains less thermal energy. Therefore, objects and systems can either gain or lose heat. They technically can not gain or lose cold, though, as there is no separate “cold energy.” In a thermal system, the warmer object loses heat energy to the cooler object until equilibrium is achieved.
The concept of thermal equilibrium is most often used and studied in the field of chemistry, particularly in the study of thermodynamics. Thermodynamics is usually taught at an introductory level in high school chemistry courses, but it is a very complex topic that is not fully explained to students until advanced chemistry and physics classes in college.
Along with its place in the lab, thermal equilibrium has a very important place in nature. Chemical systems exist in many living and nonliving things in nature, and keeping those systems in a state of equilibrium is very important. In the human body, for example, enzymes are involved in countless chemical reactions that are vital to life. Enzymes, however, become useless when they reach too high or too low of a temperature. They must remain at a constant equilibrium to remain functional, and they must remain functional for the human body itself to function.