Question

What happens to the entropy of a pure substance during a phase change at constant temperature and pressure?

• A. It decreases.
• B. It remains constant.
• C. It increases.
• D. It becomes zero.

Hint:

Phase changes from a more ordered state to a less ordered state (solid $\to$ liquid $\to$ gas) are accompanied by an absorption of latent heat, which increases the molecular disorder and hence the entropy.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This question focuses on the change in entropy of a substance when it undergoes a phase change (such as vaporization or melting) at constant temperature and pressure.

Step 2: Key Formula or Approach:
The change in entropy ($dS$) during a reversible phase change is given by:
\[ \Delta S = \frac{Q_{\text{latent}}}{T} \]
where $Q_{\text{latent}}$ is the latent heat transferred during the process, and $T$ is the constant absolute temperature.

Step 3: Detailed Explanation:

• During typical phase changes like melting (solid to liquid) or boiling (liquid to gas), heat is added to the system ($Q_{\text{latent}} > 0$).

• Since heat is supplied to overcome intermolecular attractive forces, the molecular disorder of the system increases.

• Substituting a positive heat value into the entropy equation ($\Delta S = Q / T$) results in a positive entropy change ($\Delta S > 0$).

• Hence, the entropy of the substance increases during melting or boiling.

• Although condensation or freezing would decrease entropy, standard textbook questions generally assume the phase change direction of increasing temperature (melting/evaporation) unless specified otherwise.

Step 4: Final Answer:
The entropy of the substance increases during this phase change process.