Thermodynamic Equilibrium

Thermodynamic equilibrium, in simple words, is the same temperature. At same temperature bodies do exchange heat but do not gain or lose heat.

A system is said to exist in a state of thermodynamic equilibrium when no change in any macroscopic property used if the system is isolated from its surroundings. an isolated system always reaches in course of time a state of thermodynamic equilibrium and can never depart from it spontaneously.

How can an isolated system experience a change in its macroscopic properties? We know that an isolated system can't exchange matter and energy from its surrounding. How does the change happen without any external interaction?

for example, A cup of tea whose temperature is measured to be 60 degrees Celsius is kept in a room which is at 25 degrees Celsius. So, eventually, the tea's temperature is gonna drop until it reaches 25. And, then we can say that tea is in thermodynamic equilibrium with respect to the room.
But, if we isolate the tea, how can we talk about its equilibrium over the course of time? Since the tea is not interacting with any external environment, how does it's temperature drops? How does it attain thermodynamic equilibrium and with respect to what?

Thermodynamics studies mainly the properties of physical systems that are found in equilibrium states.

A system will be in a state of thermodynamic equilibrium if the conditions for the following three types of equilibrium are satisfied.
  1. Mechanical Equilibrium
  2. Chemical Equilibrium
  3. Thermal Equilibrium
In the absence of an unbalanced force within the system itself and also between the system and the surroundings, the system is said to be in a state of mechanical equilibrium.

If there is no chemical reaction or transfer of matter from one part of the system to another, such as diffusion or solution, the system is said to exist in a state of chemical equilibrium.

When a system existing in mechanical and chemical equilibrium is separated from its surroundings by a diathermic wall and if there is no spontaneous change in any property of the system, the system is said to exist in a state of thermal equilibrium. when this is not satisfied, the system will undergo a charge of state till thermal equilibrium is restored.

When the conditions for any one of three types of equilibrium are not satisfied, a system is said to be in a non equilibrium state.