Answer: At equilibrium, nothing macroscopic changes (no heat/work interactions with the environment) and the system properties are constant with no change of state. d(properties)/dt=0
A system is said to be in equilibrium when it attains:
Thermal Equilibrium
Mechanical Equilibrium
Phase Equilibrium
Chemical Equilibrium
1. Thermal Equilibrium:
Temperature is the same throughout the entire system.
2. Mechanical Equilibrium:
This equilibrium is related to pressure and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time.
3. Phase Equilibrium:
Phase equilibrium is achieved in multiphase systems when the mass of each phase reaches an equilibrium and stays there.
4. Chemical Equilibrium:
The chemical composition of the system does not change with time (No chemical reactions occur after equilibrium is achieved)
Strictly speaking, equilibrium does not exist in nature. When the universe reaches a state of equilibrium, the universe would have achieved a state of maximum entropy and it is called as the 'Heat Death' of the universe.
[Don't worry, we are nowhere close to that state :) ]