Following statements consider single pure substance. Which of them is/are true?

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BlackTom AI · Accepted Answer

The question asks which statements about a single pure substance undergoing phase change are true, so we evaluate each option carefully.
Option a: 'There are liquid droplets in superheated steam' — By definition, superheated steam is steam that has been heated beyond its saturation temperature at a given pressure, meaning it is in the gaseous phase with no liquid droplets present. If liquid droplets were present, the mixture would be moist or saturated vapor, not truly superheated steam. Therefore, this statement is false.
Option b: 'If vaporization occurs at constant pressure, temperature increases' — During a phase change at a given pressure (such as boiling), the temperature remains essentially constant, equal to the saturation temperature corresponding to that pressure, until all the liquid has vaporized. Saying the temperature increases during vaporization contradicts the fundamental boiling principle. Hence, this statement is false.
Option c: 'If vaporization occurs at constant pressure, also temperature is constant' — This aligns with the concept of boiling: at a fixed pressure, the liquid boils at a constant temperature (the saturation temperature for that pressure) and remains at that temperature throughout the phase change. Thus, this statement is true.
Option d: 'Wet/moist steam contains saturated liquid and saturated steam' — Wet or moist steam is indeed a mixture comprising saturated liquid droplets dispersed in saturated vapor. It is a characteristic description of a two-phase mixture on its way to becoming fully vapor. Therefore, this statement is true.
In summary, the false statements are a and b, while the true statements are c and d, reflecting the behavior of a pure substance during vaporization at constant pressure and the nature of wet steam.

BH20A0720 Engineering Thermodynamics - Blended teaching 2.9.2024-15.12.2024 3rd full exam 29.04.2025 at 16:30-19:30

Following statements consider single pure substance. Which of them is/are true?

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