Thermodynamic equilibrium describes a system whose properties will not change without some sort of outside interference. In other words, a system in thermodynamic equilibrium will not change unless something is added or subtracted from it. An example of this is a lukewarm beverage. The beverage may have started out cold, but the heat from the air will move into the cold drink and make it warmer until it is the same temperature as the air—thus reaching thermodynamic equilibrium. In addition, because heat is moving from the air into the beverage, the air immediately around the cu\p will also become cooler.
The process a system uses to reach thermodynamic equilibrium is described in two laws of physics: the First Law of Thermodynamics and the Second Law of Thermodynamics. The first law states that energy cannot be created or destroyed, it can only be transferred. The second law says that in an isolated system, entropy will increase until equilibrium is reached. This is the essence of thermodynamic equilibrium. For an object to reach thermodynamic equilibrium, three conditions have to be fulfilled: chemical equilibrium, mechanical equilibrium, and thermal equilibrium.
Mechanical equilibrium describes what happens when there are no unbalanced forces in a system or with a system and its surroundings. This means that the forces must be equal in a system and in a system and its surroundings. One such force is pressure. If the pressure is the same in the system and with the system and its surroundings, mechanical equilibrium is reached. If there is no mechanical equilibrium, the system will endeavor to reach equilibrium.
For a system to be at chemical equilibrium, no net chemical reactions should be going on. In some systems this can mean that chemical reactions have stopped. However, in other systems this can mean that a system has reached dynamic equilibrium. Dynamic equilibrium describes a state in which forward and reserve reactions are happening in such a way that the net amount of the reactants remain unchanged. In chemical equilibrium, it is also necessary that matter should not be moving from one area to another, such as what happens when diffusion is taking place.
When an object is at thermal equilibrium, the temperatures must be the same. The example of the lukewarm beverage is an example of a system reaching thermal equilibrium. When an object comes in thermal contact with another object, such as air, heat will move from higher concentration to lower concentration--that is, from hot to cold. Incidentally, this means that ice does not cool down a drink but rather the drink heats up the ice. The heat will continue to move from high concentration to low until both objects are at the same temperature and thermal equilibrium is reached.