Four different Entropy (S) - Temperature (T) diagrams, representing liquid to vapour phase transition process of a pure substance in a closed system under constant pressure are shown. The diagram, which correctly represents the process, is:
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In an entropy-temperature diagram, a vertical line indicates a phase change under constant pressure, where the temperature remains constant but the entropy increases during the transition from liquid to vapour.
The phase transition from liquid to vapour at constant pressure is represented by a vertical line in the entropy-temperature (S-T) diagram, indicating a change in entropy at a constant temperature during the phase change. The phase change occurs along the boundary between liquid and vapour phases.
Step 1: Analyze the diagrams.
- In the S-T diagram, the vertical line represents the phase transition, and the slope of the line reflects the heat required for the phase change.
- The process of liquid to vapour phase transition occurs isothermally at the boiling point of the substance, which should be depicted as a vertical line (constant temperature) where entropy increases.
Step 2: Identify the correct diagram.
- Diagram 1 correctly represents the liquid to vapour phase transition with a vertical line indicating the isothermal phase transition, where entropy increases.
- Diagrams 2, 3, and 4 do not represent the phase transition correctly as they do not depict a vertical line, which is characteristic of the transition under constant pressure.
Step 3: Conclusion.
Thus, the correct diagram that represents the liquid to vapour phase transition at constant pressure is diagram 1. The answer is (A).
