Now, let us understand another important term that is thermodynamic equilibrium and non-equilibrium. Let us consider an example. There is a cylinder and it is fitted with a piston. There is a cylinder. It is fitted with a piston and inside of which there is gas. These are the gas. Now if the piston is not moving then the temperature and different other properties of the gas will remain same. Then that is called thermodynamic equilibrium. That means the different properties of the gas inside the cylinder should remain the same or it should not change with time. That is if the properties of the system does do not change with time. It is called thermodynamic equilibrium. It is called to be in thermodynamic equilibrium. Thermodynamic equilibrium. I write equilibrium as EQM. Okay. So, for a system to be in thermodynamic equilibrium it must satisfy different criteria. There are three criteria that must be satisfied by the system in order to be in thermodynamic equilibrium. The first one is thermal equilibrium. Thermal equilibrium. That means the system must have thermal equilibrium in order to be in thermodynamic equilibrium. Thermal equilibrium means the temperature in the system should remain same in any point of the system. Okay. That is the temperature of this region and this region should be same. The temperature should not change. Okay. The temperature of the temperature of the substance, substance in this particular case gas should remain the same at all point and heat should not be and heat should not be exchanged, exchanged with surrounding. With surrounding obviously if the heat is being exchanged with the surrounding then the temperature inside the system will obviously fluctuate. Right. So, what I am trying to say is that thermal equilibrium is the condition of the system in which it has a same temperature throughout the system at all the time. Okay. So, this is the temperature of the system and the temperature of the surrounding. It is the temperature of the system and the temperature of the surrounding, they will remain same or they will remain same. That is why they want exchange heat energy. Okay. The second one is mechanical energy, sorry mechanical equilibrium, mechanical equilibrium. This is the second. This is the second criteria for a system to that must be followed in order to be in thermodynamic equilibrium. Now mechanical equilibrium is related with pressure as well. Now, let us consider a condition. This is the piston and we suddenly push the piston. This is the piston we suddenly push the piston. What will happen? The pressure inside it will obviously change. Right. And the gas near the piston will feel more pressure in first and they will get more heated. But after some time, if we keep leaving that after some time, the pressure in the temperature inside that will remain constant now. Now that condition is called mechanical equilibrium. Even if the pressure changed after some time, it is now in equilibrium that or pressure at all the point will remain same. So that condition is called mechanical equilibrium. That means if the pressure in the system remains constant, then it is called in mechanical equilibrium. Now, let us, the third one, the third criteria is chemical equilibrium. Chemical equilibrium. Now let us consider inside the system, there are many chemical species or gas gaseous, obviously. Now they are reacting with each other. Then obviously there will be forward reaction. But there should be backward reaction as well. That means if in a system there is forward reaction and backward reaction happening on. And if the rate of forward reaction and backward reaction is equal, then that system is called in, it is said to be in chemical equilibrium. If rate of forward and backward reaction is same, same. So if the rate of forward and backward reaction is same, then that system is said to be in chemical equilibrium. And to be in thermodynamic equilibrium, the system must follow all three equilibrium. That means it must have thermal equilibrium, that is the temperature should not change. Mechanical equilibrium, that is the pressure should not change. And chemical equilibrium, that is the rate of forward reaction and backward reaction will always be same. So these are the three criteria that must be followed by a system in order to be a in thermodynamic equilibrium. Now non thermo, sorry, thermodynamic non equilibrium condition is that in which any of this equilibrium does not exist. If the temperature is changing, then that is non equilibrium. If the pressure is changing, that is also non equilibrium. And if chemical like if rate of forward reaction and backward reaction is not same, that is also in non equilibrium condition. So this is what thermodynamic equilibrium and non equilibrium condition actually is.