Question

Entropy of a substance remains constant during a/an __________ change.

Answer 1

Hint: As we know that entropy of a substance or system is defined as a measure of randomness or disorder in the system. It is used to define the behavior of a substance or system in terms of thermodynamic properties such as pressure, temperature and heat capacity. So here we have to tell the type of change for which the entropy of a substance remains constant.

Complete answer: Let us begin with the discussion of entropy as follows:-
Entropy: It is defined as a measure of randomness or disorder in the system. It is used to define the behavior of a substance or system in terms of thermodynamic properties such as pressure, temperature and heat capacity. When entropy of a substance remains constant for a change, it is known as isentropic condition.
-The mathematical formula of change in entropy is:-
$\Delta S=\dfrac{q}{T}$
where,
S = entropy
q =heat exchange
T = temperature
-Entropy change remains constant during adiabatic reversible change as shown below:-
For an adiabatic process, the heat exchange is zero which means q = 0
Therefore change in entropy ($\Delta S$) = 0
So we can say that reversible adiabatic change is an isentropic process as it is an idealized thermodynamic process in which the work transfers of the system are frictionless and it is reversible.
-Hence, entropy of a substance remains constant during a reversible adiabatic change.

Note:
-Remember that for irreversible adiabatic change, the entropy keeps on increasing with the process.
-Also entropy is a thermodynamic state function which depends on the state of the system rather than the path being followed.