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Physics · Thermodynamics

Reversible & Irreversible Processes

A reversible process is an idealisation — one carried out infinitely slowly, through a chain of equilibrium states, with no friction or other dissipation, so that both system and surroundings can be returned to their starting states leaving no trace anywhere. Every real, spontaneous process in nature falls short of this and is irreversible. NCERT §11.10 builds the concept precisely because the reversible process is the benchmark of maximum efficiency that the Carnot engine attains. This deep-dive defines both, lays out the causes of irreversibility, and shows why reversibility is an idea you must hold even though you can never quite reach it.

The reversal question

Picture any process in which a thermodynamic system moves from an initial state $i$ to a final state $f$, absorbing heat $Q$ from its surroundings and doing work $W$ on them. NCERT poses one sharp question: can we reverse this process and bring both the system and the surroundings back to their initial states with no other effect anywhere in the universe? Experience says no — for most processes in nature, the answer is a flat refusal.

The base of a vessel taken off an oven is hotter than its rim; heat flows from base to rim until the whole vessel is at one temperature, but a part of it will never cool itself spontaneously to re-heat the base. Cooking gas leaking from a cylinder diffuses through the room; it will not gather itself back into the cylinder. A blade stirred in a liquid warms it; the liquid will not spontaneously spin the blade back. Each of these runs one way only. The Second Law of Thermodynamics is precisely the statement that forbids the reverse. This one-directionality is what the words "reversible" and "irreversible" pin down.

Quasi-static — the slow-motion idealisation

Before reversibility can be defined, the system must always have a well-defined state. When the external pressure on a piston is reduced suddenly, the piston accelerates outward and the gas passes through states that are not equilibrium states — they have no single well-defined pressure or temperature. The same happens when a finite temperature difference drives a rapid heat exchange. Such non-equilibrium states cannot be plotted as points and are awkward to handle.

To escape this, NCERT introduces the quasi-static process (literally "nearly static"): an idealised process so slow that at every stage the system stays in thermal and mechanical equilibrium with its surroundings. The external pressure differs from the system's pressure only infinitesimally; the reservoir temperature differs from the system's temperature only infinitesimally. The variables $P$, $V$, $T$ change so gradually that every intermediate stage is itself an equilibrium state. A quasi-static process is a hypothetical construct, but a process that is genuinely slow — no accelerated piston, no large temperature gradient — is a reasonable approximation to it.

Quasi-static (slow) gas remove sand grains one at a time → always in equilibrium P_gas ≈ P_ext at every instant Sudden (fast) gas M drop one big weight → piston accelerates non-equilibrium: no defined P, T

Figure 1. Compressing a gas grain-by-grain (left) keeps it in equilibrium at every instant — quasi-static. Dropping one large weight (right) makes the piston accelerate, driving the gas through non-equilibrium states with no well-defined pressure or temperature. Only the slow route can be a candidate for reversibility.

What makes a process reversible

NCERT's definition is exact: a thermodynamic process (state $i \to$ state $f$) is reversible if it can be turned back so that both the system and the surroundings return to their original states, with no other change anywhere else in the universe. From the discussion above, this is an idealised notion, achievable only when two conditions hold together.

  • It is quasi-static — the system stays in equilibrium with its surroundings at every stage, so the path is a continuous chain of equilibrium states that can be traced backward.
  • There are no dissipative effects — no friction, viscosity, turbulence or other mechanism that degrades ordered energy into internal energy. Dissipation always generates heat that cannot be fully recovered on the return trip.

The textbook example is a quasi-static isothermal expansion of an ideal gas in a cylinder fitted with a frictionless movable piston, kept in contact with a single large reservoir. Both conditions hold, so it is reversible. Note the word "frictionless" carries the second condition — drop it and the process, however slow, becomes irreversible.

Why nature is irreversible

NCERT states the verdict bluntly: the spontaneous processes of nature are irreversible, and "irreversibility is a rule rather than an exception in nature." A process is irreversible when it cannot be retraced through the same equilibrium states from final back to initial without leaving some change in the universe. The reason is that reversing a spontaneous process would, in effect, demand converting heat entirely into work or making heat flow unaided from cold to hot — both barred by the Second Law.

Consider the stir-the-liquid example. Stirring a liquid in thermal contact with a reservoir converts the work done into heat and raises the internal energy of the reservoir. To reverse it exactly, the reservoir would have to give back that heat and spontaneously turn it wholly into the work of un-stirring — a complete conversion of heat into work with no other effect, which the Second Law forbids. The same logic condemns combustion of a petrol–air mixture, the diffusion of a gas through a room, and the free expansion of a gas into vacuum. Each is a one-way street.

The two causes of irreversibility

NCERT traces irreversibility to two distinct roots. Sorting any real process into one or both of these is the cleanest way to argue that it is irreversible.

Cause 1

Non-equilibrium states

  • The process drives the system through states that are not equilibrium states, with no well-defined $P$ or $T$.
  • Happens whenever change is fast or uncontrolled.
  • Free expansion of a gas into vacuum.
  • An explosive chemical reaction (petrol–air ignition).
  • Rapid heat exchange across a finite temperature difference.
Cause 2

Dissipative effects

  • Friction, viscosity and similar effects turn ordered mechanical energy into internal energy (heat).
  • A moving body coming to rest, losing its kinetic energy as heat to the floor.
  • A rotating blade in a liquid stopping due to viscosity, warming the liquid.
  • Present everywhere; can be minimised but never fully eliminated.
  • This is why nearly every practical process is irreversible.

The mixing of gases, the diffusion of leaked cooking gas, and heat flowing from a hot vessel base to its cooler rim all belong here too — the first two by non-equilibrium spreading, the third by heat transfer across a finite temperature gap. Because dissipation pervades the real world and finite gradients are unavoidable in any process that runs at a finite rate, the reversible process remains an unreachable limit.

Reversible vs irreversible at a glance

FeatureReversible processIrreversible process
RateInfinitely slow (quasi-static)Finite rate; often fast or sudden
Intermediate statesAll equilibrium statesPass through non-equilibrium states
DissipationNone — no friction, viscosity, turbulenceFriction, viscosity and other losses present
Retraceable?Yes — exactly, leaving no trace anywhereNo — cannot be retraced without a net change
P–V representationContinuous solid curve through defined statesOnly end points defined; dashed link between
Occurrence in natureIdealised; never fully achievedAll spontaneous, real processes
ExamplesQuasi-static isothermal expansion with a frictionless pistonFree expansion, combustion, diffusion, stirring
Efficiency roleSets the maximum possible efficiencyAlways lower than the reversible limit

Reversible and irreversible paths on P–V

On an indicator (P–V) diagram, a reversible process is a continuous solid curve, because every intermediate point is a genuine equilibrium state with definite pressure and volume. An irreversible process cannot be drawn this way — its intermediate states are non-equilibrium and have no single $P$ to plot. Only the initial and final equilibrium states are well-defined; convention joins them with a dashed line as a reminder that the path itself is undefined.

P V i f reversible (solid: equilibrium states) irreversible (dashed: path undefined)

Figure 2. Same endpoints $i$ and $f$, two depictions. The reversible process (teal, solid) traces a continuous curve of equilibrium states and can be run backward exactly. The irreversible process (coral, dashed) has no plottable intermediate states; only $i$ and $f$ are real, so the link is drawn dashed to flag the undefined path.

Why reversibility is the efficiency ceiling

The reason thermodynamics labours over an idea that never occurs in pure form is efficiency. A central concern of the subject is how much heat can be turned into work. The Second Law already rules out a perfect heat engine of 100% efficiency. The natural next question — what is the highest efficiency possible for an engine working between reservoirs at temperatures $T_1$ and $T_2$ — has a clean answer: an engine built entirely from reversible processes attains it.

Irreversibility is bound up with dissipative effects, and dissipation always lowers efficiency. Any engine carrying friction or finite-rate heat transfer — that is, every practical engine — must therefore run below the reversible limit. The reversible engine is the unbeatable benchmark. This is exactly the engine Sadi Carnot conceived: heat absorbed isothermally at $T_1$, rejected isothermally at $T_2$, with the temperature changes carried by reversible adiabatic steps so that no heat crosses a finite temperature gap. Reversibility is thus not an abstraction for its own sake — it is the standard against which the performance of every real machine is measured.

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Go deeper

The reversible engine is realised concretely in the Carnot engine and its efficiency — two isothermals and two adiabatics, with $\eta = 1 - T_2/T_1$.

Quick recap

Reversibility in one breath

  • A reversible process returns both system and surroundings to their initial states with no other change anywhere — an idealisation.
  • Reversible requires two conditions together: quasi-static (equilibrium at every stage) and no dissipation (no friction/viscosity/turbulence).
  • Quasi-static means only infinitesimal $P$ and $T$ differences from the surroundings at every stage; "slow" alone is not enough.
  • All spontaneous, natural processes are irreversible — irreversibility is the rule, not the exception.
  • Two causes of irreversibility: (1) passage through non-equilibrium states, (2) dissipative effects that can be minimised but never eliminated.
  • Free expansion, combustion, diffusion, mixing and stirring are all irreversible; heat flow across a finite temperature difference is too.
  • A reversible engine sets the maximum possible efficiency between two temperatures — the Carnot benchmark.

NEET PYQ Snapshot — Reversibility & Work

Reversibility rarely stands alone in NEET; it underpins the engine and work comparisons. Two PYQs where the idealised reversible/quasi-static framing decides the answer.

NEET 2016

A gas is compressed isothermally to half its initial volume. The same gas is compressed separately through an adiabatic process until its volume is again reduced to half. Then:

  1. Compressing the gas through adiabatic process will require more work to be done.
  2. Compressing the gas isothermally or adiabatically will require the same amount of work.
  3. Which case requires more work depends upon the atomicity of the gas.
  4. Compressing the gas isothermally will require more work to be done.
Answer: (1)

Reversible-path logic. Both compressions are taken as quasi-static (reversible) so each is a solid P–V curve. To the same final volume $V/2$, the adiabatic curve lies above the isothermal one ($P \propto V^{-\gamma}$ versus $P \propto V^{-1}$, with $\gamma>1$). Greater area under the adiabatic curve means more work must be done on the gas. The comparison is only meaningful because both paths are reversible and therefore plottable.

NEET 2025

Two gases A and B are filled at the same pressure in separate cylinders with movable pistons of radius $r_A$ and $r_B$. On supplying an equal amount of heat to both systems reversibly under constant pressure, the pistons of A and B are displaced by 16 cm and 9 cm respectively. If the change in their internal energy is the same, then $r_A/r_B$ equals:

  1. $\sqrt{3}/2$
  2. $4/3$
  3. $3/4$
  4. $2/\sqrt{3}$
Answer: (3) 3/4

Reversible isobaric work. The word "reversibly" lets us write $W = P\,\Delta V$ as a definite quantity. First law: $\Delta Q = \Delta U + W$. Equal $\Delta Q$ and equal $\Delta U$ force $W_A = W_B$, so $(P\,\Delta V)_A = (P\,\Delta V)_B$. With equal $P$: $\pi r_A^2 d_A = \pi r_B^2 d_B \Rightarrow r_A/r_B = \sqrt{d_B/d_A} = \sqrt{9/16} = 3/4$.

FAQs — Reversible & Irreversible Processes

The reversibility questions NEET aspirants most often get wrong.

Is every quasi-static process reversible?
No. Quasi-static is necessary but not sufficient. A process is reversible only if it is BOTH quasi-static (passing through equilibrium states) AND free of dissipative effects such as friction, viscosity and turbulence. A gas compressed infinitely slowly but through a piston with friction is quasi-static yet irreversible, because the frictional heat cannot be recovered on the return path.
Why are all spontaneous natural processes irreversible?
A spontaneous process proceeds on its own toward equilibrium — heat flowing from hot to cold, a gas diffusing through a room, a stirred liquid warming. Reversing any of these without leaving a trace elsewhere would amount to extracting heat and converting it wholly into work, which the Second Law of Thermodynamics forbids. NCERT puts it plainly: irreversibility is a rule rather than an exception in nature.
Is free expansion of a gas reversible or irreversible?
Free expansion is irreversible. When a gas rushes into an evacuated chamber it passes through non-equilibrium states with no well-defined pressure or temperature, and it does no work. The gas will never spontaneously collect itself back into the original half. It fails the quasi-static condition outright, so it cannot be a reversible process.
If no real process is reversible, why study reversibility at all?
Because the reversible process is the idealised limiting case that fixes the ceiling on performance. A heat engine built entirely from reversible processes achieves the highest efficiency possible between two given temperatures; every real engine, carrying friction and finite-rate losses, falls below it. The Carnot engine is exactly this reversible benchmark, so reversibility is the yardstick against which all real machines are measured.
Can an irreversible process be shown as a path on a P–V diagram?
Not as a continuous solid curve. During an irreversible process the system passes through non-equilibrium states that have no single well-defined pressure or temperature, so there is no point to plot. Only the initial and final equilibrium states are definite; they are usually joined by a dashed line to signal that the in-between path is undefined. A reversible process, being a chain of equilibrium states, is drawn as a solid curve.
What are the two main causes of irreversibility?
NCERT identifies two. First, many processes — free expansion, an explosive reaction — drive the system through non-equilibrium states. Second, most processes involve dissipative effects such as friction and viscosity that convert ordered mechanical energy into internal energy. Dissipation is everywhere and can be minimised but never fully eliminated, which is why nearly all practical processes are irreversible.
Related Topics
Thermodynamics — Parent chapter Full chapter map: laws, processes, engines and refrigerators. Second Law of Thermodynamics Kelvin–Planck and Clausius statements; why reversal is forbidden. Carnot Engine & Efficiency The reversible benchmark; η = 1 − T₂/T₁; Carnot's theorem. Thermodynamic Processes Quasi-static, isothermal, isobaric, isochoric, adiabatic. Heat Engines Source, sink, working substance; efficiency basics. First Law of Thermodynamics ΔQ = ΔU + W; energy conservation in processes.