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Notes on Entropy

Entropy is a state property frequently referred to as a system's state of disorder. The 2nd and 3rd laws of thermodynamics are both based on entropy.

Entropy is a qualitative measure of the amount of energy of subatomic particles spreading out in a process, and it can be expressed in terms of a system’s statistical probability or other thermodynamic characteristics. Entropy is a thermodynamic variable used to describe the path of a process, such as whether it is a spontaneous process with a likelihood of occurring in a specific direction or perhaps a non-spontaneous process with a probability of occurring oppositely. For many daily occurrences, the notion of entropy gives a deeper understanding of the path of spontaneous change.

All About Entropy

Rudolf Clausius

Rudolf Clausius proposed a new thermodynamic characteristic that connects the natural heat flow that occurs during a procedure to the temperature. The ratio of reversible heat (qrev) to kelvin temperature was used to express this new feature (T). Like other thermodynamic properties, this new quantity is a state function, which means that its change is solely dependent on the system’s initial and final states. Clausius identified this attribute entropy (S) in 1865 and characterised its variation for any process as:

The entropy change is equal when a real, irreversible process has the same initial and ultimate states as a theoretical reversible process.

Comparison between entropies

The atoms or molecules in the solid phase are constrained to relatively steady positions about each other and can only make minor oscillations around these positions. The atoms or molecules in the liquid phase can move around and through each other while remaining relatively near to one another. As a result of the enhanced flexibility of motion, the number of possible particle sites increases. Consequence, Sliquid > Ssolid, and the process of turning a solid into a liquid (melting) is accompanied by a rise in entropy,  ΔS > 0. According to the same argument, the reverse process (freezing) has a lower entropy, ΔS < 0.For any substance, Sgas > Sliquid > Ssolid

Factors affecting entropy

Structure of particles

The arrangement of the components (atoms or molecules) that make up a substance impacts its entropy. In atomic substances, heavier atoms have more entropy at a specific temperature than lighter atoms due to the relationship between a particle’s mass and the separation of quantised translational energy states. When there are more atoms in a molecule, the number of ways the molecules can oscillate increases, as does the amount of potential microstates and the system’s entropy.


The temperature of a particle is proportional to the mean kinetic energy of its particles, according to kinetic-molecular theory. Rising the substance’s temperature causes the particles in solids to vibrate more widely and the molecules in liquids and gases to translate more quickly. The dispersion of kinetic energy among some of the individual particles of the element is also wider (more scattered) at higher temperatures than at lower temperatures. As a result, the entropy of any substance rises as the temperature rises.

Vibration of a particle

The entropy of a species is affected by variations in particle characteristics. The entropy of a combination of two or more separate particle types is larger than that of a pure substance wherein all particles are identical. This is due to the various alignments and interactions that a system made up of nonidentical components can have. As a result, the phase of dissolution contains a rise in entropy, ΔS > 0.

Examples of entropy

As a slab of ice melts, its entropy increases. It’s clear to see how the system’s instability is increasing. Ice is made up of water molecules linked together in a crystalline structure. When ice melts, particles gain energy, spread out more, and lose structure, resulting in a liquid. Similarly, changing from a liquid to gas increases the system’s energy, such as liquid to vapour.

On the other hand, energy can be depleted. This happens when steam turns into water or when water turns into ice. Because the stuff is not in a closed system, the second law is not broken. Although the system’s entropy under consideration may be decreasing, that of the environment is increasing.


The number of microstates for a structure (the number of methods the system can be configured) and the proportion of bidirectional heat to kelvin temperature are connected to entropy (S). It is often stated as indicating the “disorder” of a system, and it can be viewed as a measurement of the distribution or dispersion of material and/or energy in a system.

Entropy is often higher for heavy nuclei or more complex molecules in a given substance,  Sgas > Sliquid > Ssolid  in a particular physical state at a specific temperature. When a body is heated or when a solution develops, entropy rises. The direction of entropy variations for some chemical processes can be reliably predicted using these rules.


Frequently asked questions

Get answers to the most common queries related to the IIT JEE Examination Preparation.

What is Entropy?

Ans. Entropy is the measure of a thermodynamic system’s randomness or disorder.

Write down the S.I unit of Entropy.

Ans. The S.I unit of Entropy is Joules per Kelvin.

What is the relation between Temperature and Entropy?

Ans. Temperature and Entropy are Directly Proportional. A rise in temperature leads to an increase in entrop...Read full

Which has the highest entropy among solids, liquids and gases?

Ans. Gases have the highest entropy.