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Reversible and Irreversible Processes

Reversible and Irreversible processes - Definition, Difference between Reversible and Irreversible processes, Practice Problems and FAQs

Don’t you feel like cuddling this cute little turtle. It's so adorable! I wish it would not have to face the harsh life here on earth. But you know what, no matter what happens, this turtle cannot go back to its shell in the same packing. So, can we say its arrival is an irreversible process? Well! Let's see first, what is a process. 

So many processes keep happening all around us such as the burning of a matchstick, rusting of iron, etc., and not to forget, inside us too like the process of our own ageing. In all these forms we see that an entity goes from an initial state to a final state. We can also call the process a change of state. Well, in thermodynamics, when a system goes from one state to another, we say that it is undergoing a process. 

Now, there are infinite different ways to get a process done. But interestingly, these processes can be broadly classified into two types based on the way they are done. What are the ways? We will see. 

In all of these processes, we can see that the system in question progresses from an initial state to a final state in which the system absorbs some heat from the environment and does some work on it. Now, for how many such systems can the system and the surrounding be brought back to their initial state with the same amount of heat being rejected to the surroundings and the same amount of work being done on the system? We can easily say that it is not possible in most cases. Let’s learn more about the type of processes and what kind of processes are found in nature and in the aspects of thermodynamics that are reversible and irreversible.


What is a Reversible Process?

A thermodynamic process (initial state → final state) is said to be a reversible process if the process can be reversed in such a way that both the system and the surroundings return to their initial states, without any net change in the universe. 

Let’s assume there is some amount of gas in a cylinder-piston system. 

The piston is assumed to be massless and frictionless and for now, the piston is also stationary.

This means, the inside and outside pressure are the same, or we can say that the system is in equilibrium i.e., Pext = Pint (gas)

Now let’s say if we compress this gas from its initial volume V1 to a final volume V2. This process can be done in two different ways. Let’s look at the first one.

Reversible compression of a Gas

Here, we put grains of sand on top of the piston and thereby increase the external pressure on the gas. Notice that the piston is going down and the gas is getting compressed. The movement of the piston is barely visible to our naked eye, let’s zoom in and view this compression.

Let the very tiny extra pressure due to this grain that assists the compression to be denoted as dp. This infinitesimally small increase in pressure is due to the extra force acting due to the grain of sand of mass dm. Of course, you know that this extra force will be: 


dm is the mass of each tiny sand grain

g is the acceleration due to gravity

And we know the pressure is force/area. So, 


Where, A is the cross-sectional area of the piston.

To continue with the compression, we need to keep adding more sand particles and the addition of each new particle can be considered as a new step in the compression process. After each step, the piston moves down slightly, and the gas system responds by increasing its own internal pressure due to compression and coming to equilibrium with its surroundings or we can say that after every step (after the addition of every grain of sand), the external pressure becomes equal to the internal pressure of the gas.


Where Pint is the internal pressure of the gas.

One important thing to note here is that, since the system is in equilibrium, it can now move in either direction after each step depending on the external conditions, right?

What does it mean is that we could reverse the process direction at any point in time by reversing the driving force behind it? Like, if we remove one sand particle after a step, the gas would expand by a small amount instead of getting compressed, thereby going back to the volume from our previous step. 

So, a thermodynamic process is said to be a reversible process if the process can be turned back in such a way that both the system and the surroundings return to their original states, with no other change anywhere else in the universe. In our case also, if the sand grains are added and removed, the volume change will be of the same amount both in cases of compression and expansion respectively without changing the universe conditions. 

So, let's think in this way if 30 J of work is done on the system from the surrounding to compress the gas then if the process is reversible, then the same amount of work will be done by gas on surrounding while expansion.

As we all know, there are no such things as reversible processes in reality. Reversible processes can thus be simply characterised as idealizations or models of real processes based on which the system or device's boundaries are to be specified. They assist us in determining the maximum efficiency a system can deliver under ideal operating conditions, as well as the target design that can be established.

Examples of Reversible Processes

  • Compression and extension of springs
  • Infinitesimally slow isothermal compression or expansion of gases
  • Melting of Ice to water and Cooling of water to Ice 
  • Movement of objects without experiencing friction from surface
  • Infinitesimally slow adiabatic compression or expansion of gases

What is an Irreversible Process?

An irreversible process can be defined as a process in which the system and the surroundings do not return to their initial state once the process is started. In other words, in an irreversible process, if the system and surroundings return to their original state after taking certain paths, there will be some net change which occurred in the universe. Consider the case of a vehicle engine that has travelled a distance using gasoline in the amount of 'x'. The fuel burns to generate energy to the engine, converting itself to smoke and heat energy in the process. We will never be able to recover the energy lost by the fuel, nor will we ever be able to return to its original state. 

The irreversibility of a process is caused by a number of reasons, including:

1. Friction is the process of converting the energy of the fuel into heat energy.

2. The unregulated expansion of the fluid prevents the fuel from returning to its original form. The reversal of heat transfer through a finite temperature is not feasible because the forward process is spontaneous in this scenario.

3. Intermixing two different substances can't be separated since the intermixing process is spontaneous in nature, and the opposite isn't possible.

As a result, depending on their ability to return to their original form from their final state, certain processes are reversible while others are irreversible in nature.

Illustration for an irreversible process

Considers an ideal gas enclosed in a cylinder at 10 atm. At any point in time, there are two forces acting on the piston.

Upward force: It acts due to the collisions of the gas particles with the piston i.e., the internal pressure of the gas. .

Downward force: It acts due to the atmospheric pressure i.e., the external pressure. 

The piston is moved upward quickly to a new position such that the volume enclosed by the piston becomes double the initial volume. During this expansion process, a new zone is created near the piston where the gas particles are not uniformly distributed. The pressure at any two points in the new volume is different, or we can say that the pressure is not defined for the system in this intermediate stage because the system is not in thermodynamic equilibrium. After some time, let assume when the gas particles are equally distributed in the cylinder, then, according to Boyle's law, the pressure of the final state is 5 atm. Thus, the pressure can now be defined for this final state, and now the system is in thermodynamic equilibrium. During this whole process of expansion, the pressure of the system is defined only when it is in thermodynamic equilibrium and not in an intermediate stage.

So, it can be considered an irreversible process that this process is a fast process and it can not be reversed without affecting the system or surroundings. Moreover, the final pressure of the gas is equal to the external pressure. 

Examples of Irreversible Processes

  • Movement of objects with experiencing friction from surface
  • Throttling of a gas from a small hole
  • Heat transfer from one object to another 
  • Diffusion/effusion of gasses 

Difference Between Reversible and Irreversible Process

Reversible Process

Irreversible Process

Reversible processes are ideal processes

Irreversible processes are real processes

Reversible process is a slow process

Irreversible process is a fast process

Reversible process can be reversed without affecting the universe

Irreversible process cannot be reversed without affecting the universe

In a reversible process, infinite changes occur in the system.

In an irreversible process, finite changes occur in the system. 

Reversible process takes infinite time for the completion

Irreversible process takes a finite time for the completion

In the case of a reversible process, there is always an equilibrium condition establishment between the initial state and the final state of a system

In case of an irreversible process, the initial state and the final state of a system is never in the equilibrium. The equilibrium is only achieved at the end of the process. 

In the case of a reversible process, Pint= Pext throughout the process. 

In the case of an irreversible process, PintPext during the process. Internal pressure is only equal to external pressure when the equilibrium is achieved that is at the end of the process. 

Examples: Compression and extension of springs, Infinitesimally slow isothermal compression or expansion of gases, Movement of objects without experiencing friction from surface Infinitesimally slow adiabatic compression or expansion of gases

Examples: Movement of objects with experiencing friction from the surface, Throttling of a gas from a small hole, Heat transfer from one object to another, Diffusion/effusion of gasses 

Practice Problems

Q 1. Which of the following processes does not result in the system and its surroundings returning to their previous state once the operation is completed?

a.  Reversible process
b.  Adiabatic process
c.  Quasi-static process
d.  Irreversible process

Answer: (D)

In an irreversible process, the system and surroundings do not come back to their original state after the process is completed. The system and surroundings which can be brought back to their original form are known as reversible processes like melting of ice, expansion, or compression of spring. 

Q 2. Select the correct statement from the following.

a.  Work done for the reversible process depends only on the initial and final stages.
b.  Work done for the reversible process depends only on the initial stage.
c.  Work done for the irreversible process depends only on the initial and final stages.
d.  Work done for the irreversible process depends only on the final stage.

Answer: (C)

Work done for the irreversible process depends only on the initial and final stages. Irreversible processes directly return to the starting position in one step as it is a fast process. So, work done can be calculated just by knowing the initial and final volume and pressure conditions.

Q 3. Which of the following process is the infinitesimally slow process?

a. Reversible
b. Irreversible
c. Both
d. None of the above.

Answer: (A)

Reversible process is an infinitesimally slow process, but there are no such processes which are reversible just because they are slow. The reason is that the process moves infinitesimally slow and each step is added, it goes through a succession of equilibrium states. 

Q 4. Which process achieves equilibrium in each step?

a. Reversible
b. Irreversible
c. Both
d. None of the above

Answer: (A)

Reversible process is a slow process which goes through various smaller stages which maintain equilibrium between the system and the surroundings at each stage of the process. So, at all stages, Pint= Pext

Frequently Asked Questions – FAQ

Q 1. What is a quasi-static process and why quasi-static processes are slow processes? 

Answer: A quasi-static process is a process which occurs infinitesimally slow such that at every stage of the process thermodynamic equilibrium is maintained. A reversible process is said to be a quasi-static process, where the change is likely to be occurring at a highly slow rate. The rate of change is so slow that it seems to be in equilibrium at all times.

Q 2. Why the work done in the reversible process is maximum?

Answer: The work done in the reversible process is maximum, due to almost negligible loss of the heat to the surrounding. The reversible process which is infinitely slow will produce maximum work. It means that the heat energy which is released during the reversible process will do the maximum amount of work because a very less amount of energy is lost in the form of heat. 

Q 3. Are natural processes reversible or irreversible?

Answer: Natural processes are spontaneous. There is no human interference in the natural processes and the processes occur by themselves. However, if we want the change to proceed in the opposite direction, we need some external aid. Thus, all the natural processes are irreversible in nature.

Q 4. List two phenomena that are irreversible.

Answer:  Many natural occurrences are unavoidably irreversible. Below is a list of a handful of them.

  • The heat flow between two bodies occurs due to the temperature gradient from a hot body to a cold body but vice-versa is not possible. 
  • The release of Helium gas from the balloon is irreversible in nature.

Related Topics:

Isothermal Process

First Law of Thermodynamics

Thermodynamic Processes

Degrees of Freedom

Heat Capacity Cp Cv relation

Zeroth Law of Thermodynamics

Second Law of Thermodynamics

Third Law of Thermodynamics

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