Which of the processes described below are irreversible? (Note: more than one of the given options may be corr

NCERT Exemplar · Q7

Q. Which of the processes described below are irreversible? (Note: more than one of the given options may be correct.)

(a) The increase in temprature of an iron rod by hammering it.

(b) A gas in a small cantainer at a temprature

T_1

is brought in contact with a big reservoir at a higher temprature

T_2

which increases the temprature of the gas.

(d) An ideal gas is enclosed in a piston cylinder arrangement with adiabatic walls. A weight W is added to the piston, resulting in compression of gas.

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Concept understanding — Second Law of Thermodynamics

Why does anything ever stop?

Think about a hot cup of tea left on a table. You know it cools down to room temperature. You also know it never spontaneously heats back up — the room never gets colder while the tea gets hotter. That obvious fact is the heart of the Second Law.

The First Law of Thermodynamics says energy is conserved. So when the tea cools, the energy doesn't disappear — it spreads into the air. But why does it always spread out from the hot object, never in? The First Law doesn't forbid the reverse. If the air lost some heat and the tea gained it, energy would still be conserved. Yet it never happens.

That "one-way direction" is what the Second Law captures.

The intuitive picture: disorder always increases

Imagine a brand-new deck of cards, perfectly ordered by suit and rank. Shuffle it once. The order is gone. You can shuffle a million times — you will never get back to that perfect order by chance. The natural tendency is from order to disorder, not the other way around.

Heat behaves the same way. A hot object has its molecules jiggling vigorously; a cold object has them jiggling slowly. When they touch, the fast jiggling spreads to the slow jiggling — the energy "shuffles" into a more mixed, disordered state. The reverse (all the fast jiggling spontaneously gathering in one spot) would be like a shuffled deck spontaneously reordering itself. It's not impossible in principle, but it is astronomically unlikely.

Note

This is why the Second Law is often called the "arrow of time." It tells you which direction time flows: from ordered to disordered, from hot to cold, from concentrated to spread out.

The precise statement

There are several equivalent ways to state the Second Law. The most common one for beginners is the Clausius statement:

\text{Heat cannot spontaneously flow from a colder body to a hotter body.}

"Spontaneously" is the key word. You can make heat flow from cold to hot — that's what a refrigerator does. But it requires work (energy input from outside). Without that work, it never happens.

The other classic statement is the Kelvin-Planck statement:

\text{No process is possible whose sole result is the complete conversion of heat into work.}

This means you cannot build a perfect engine. Some heat must always be "wasted" — dumped into a cold reservoir. That's why every real engine has an efficiency less than 100%.

The formal quantity: entropy

To make the Second Law quantitative, we define a property called entropy ( $S$ ). For a small amount of heat $Q$ transferred at a constant temperature $T$ (in Kelvin), the change in entropy is:

\Delta S = \frac{Q}{T}

The Second Law then becomes:

Important

$\Delta S_{\text{universe}} \geq 0$

The entropy of an isolated system never decreases. It either stays the same (for a reversible process) or increases (for an irreversible process).

When the tea cools, the entropy of the tea decreases (it loses heat), but the entropy of the surrounding air increases by a larger amount. The total entropy of the universe goes up. That's why the process happens. …

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Irreversible processes are those that cannot be reversed without leaving a change in the surroundings, often involving dissipative effects or finite gradients. Options (A), (B), and (D) describe irreversible processes.

Understanding whether a process is reversible or irreversible is fundamental in thermodynamics. A process is considered reversible if it can be reversed in such a way that both the system and its surroundings return to their initial states, with no net change in either. This implies that the process must occur infinitesimally slowly (quasi-statically) and without any dissipative effects like friction, viscosity, or heat transfer across a finite temperature difference.

Conversely, an irreversible process is one that cannot be reversed without leaving some permanent change in the system or its surroundings. All real-world processes are irreversible to some extent. Key characteristics of irreversible processes include:

They occur spontaneously in a particular direction.
They involve dissipative effects (e.g., friction, inelastic deformation, electrical resistance).
They involve heat transfer across a finite temperature difference.
They involve expansion or compression against a finite pressure difference (i.e., not quasi-static).
The entropy of the universe increases during an irreversible process.

Let's analyze each option based on these principles.

Option (A): The increase in temperature of an iron rod by hammering it.
- When an iron rod is hammered, mechanical work is done on it. This mechanical energy is converted into the internal energy of the rod, leading to an increase in its temperature.
- This conversion of ordered mechanical energy into disordered thermal energy is inherently irreversible. We cannot spontaneously convert all the increased thermal energy back into the exact mechanical work of hammering without any other changes in the surroundings. Energy is dissipated as heat, sound, and permanent deformation.
- Therefore, this process is irreversible.
Option (B): A gas in a small container at a temperature $T_1$ is brought in contact with a big reservoir at a higher temperature $T_2$ which increases the temperature of the gas.
- This process involves heat transfer from the high-temperature reservoir ( $T_2$ ) to the lower-temperature gas ( $T_1$ ).
- Heat transfer across a finite temperature difference is a classic example of an irreversible process. To reverse this process (i.e., to transfer heat from the gas back to the reservoir without external intervention), it would violate the second law of thermodynamics. A refrigerator would be needed to achieve this, which itself requires work input and leaves changes in the surroundings.
- Therefore, this process is irreversible.
Option (C): A quasi-static isothermal expansion of an ideal gas in a cylinder fitted with a frictionless piston.
- The term "quasi-static" means the process occurs infinitesimally slowly, such that the system is always in thermodynamic equilibrium (or infinitesimally close to it).
- "Isothermal" means the temperature remains constant throughout the process.
- "Frictionless piston" implies no mechanical energy dissipation due to friction.
- Since the process is quasi-static and involves no dissipative effects (like friction or heat transfer across a finite temperature difference, as the temperature is maintained by a reservoir at the same temperature as the gas), it can be perfectly reversed. The gas can be compressed back isothermally by infinitesimally increasing the external pressure, returning both the gas and the surroundings to their initial states.
- Therefore, this process is reversible. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set ANNUAL1 mark
$Q. Write the second law of thermodynamics.$
›Reveal solutionSolution

The second law of thermodynamics states that heat spontaneously flows only from a hotter to a colder body, and no heat engine can convert absorbed heat completely into useful work in a cyclic process.

The first law of thermodynamics (energy conservation) alone does not forbid processes that never actually happen in nature, such as heat flowing spontaneously from a cold body to a hot one, or a heat engine converting 100% of the heat it absorbs into work with no waste heat rejected. The second law rules these out. Two equivalent standard statements: (1) Clausius statement — it is impossible for heat to flow, of itself (without external work being done), from a body at a lower temperature to a body at a higher temperature. (2) Kelvin–Planck statement — it is impossible to construct a heat engine, operating in a cycle, whose sole effect is to absorb heat from a single reservoir an …

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CBSE 2025Set ANNUAL1 mark
$Q. State the second law of thermodynamics.$
›Reveal solutionSolution

The second law of thermodynamics restricts the direction of natural processes and rules out perfect heat engines and perfect refrigerators; it can be stated in (equivalent) Kelvin-Planck or Clausius forms.

Kelvin-Planck statement: It is impossible to construct a device that, operating in a cycle, produces no effect other than the absorption of heat from a single reservoir and the performance of an equivalent amount of work — i.e. no engine can have 100% efficiency.

Clausius statement (equivalent): It is impossible for a self-acting device, unaided by any external agency, to transfer heat from a body at a lower temperature to a body at a higher temperature.

…

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CBSE 2024Set sz1 markMCQ
$Q. During the free expansion of gas, the entropy of gas will: (A) Increase (B) Decrease (C) First increases, then decreases (D) Remain same$
›Reveal solutionSolution

Free expansion is irreversible, and the second law of thermodynamics states that the entropy of an isolated system always increases in an irreversible process — so the gas's entropy increases even though its internal energy and temperature stay unchanged.

In free expansion, a gas rushes into an evacuated region with no external pressure to work against ( $W=0$ ) and no heat exchange with surroundings ( $Q=0$ ), so by the first law $\Delta U = 0$ and, for an ideal gas, temperature stays constant.

…

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CBSE 2023Set ANNUAL1 mark
$Q. State second law of thermodynamics.$
›Reveal solutionSolution
No engine can convert heat entirely into work with 100% efficiency (Kelvin-Planck statement); equivalently, heat cannot spontaneously flow from a colder to a hotter body on its own (Clausius statement).

The first law of thermodynamics is just a statement of energy conservation — it does not forbid processes that conserve energy but never actually happen in nature (like heat flowing spontaneously from a cold body to a hot one). The second law fills this gap by specifying the DIRECTION in which natural processes proceed.

Two commonly stated (and equivalent) forms:

Kelvin-Planck statement: It is impossible to construct a device (a heat engine) that, operating in a cycle, produces no other effect than the absorption of heat from a single reservoir and its complete conversion into an equivalent amount of work. In other words, no heat engine can have 100% efficiency — some heat must always be rejected to a sink. …
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CBSE 2020Set ANNUAL1 markMCQ
$Q. All natural processes occur such that entropy should:$
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›Reveal solutionSolution

By the second law of thermodynamics, entropy of an isolated system increases in every natural (irreversible) process, reaching a maximum at equilibrium.

Entropy is a measure of the disorder or randomness of a system. The second law of thermodynamics states that in any natural (spontaneous) process occurring in an isolated system, the total entropy of the system either increases (for irreversible processes, which all real/natural processes are) or stays the same (only in the idealised limit of a reversible process).

…

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Q.Which of the processes described below are irreversible? (Note: more than one of the given options may be correct.)

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