Question

Which of the following statements are correct:
A. Specific heat of saturated water vapour at 100\(^{\circ}\)C is negative.
B. There is only one triple point of a substance.
C. Boiling point of every liquid rises with increase in pressure.
D. Latent heat can not become zero.
Choose the CORRECT answer from the options given below:

• A. A, B and D only
• B. A, B and C only
• C. A, B, C and D
• D. B, C and D only

Hint:

Phase diagrams are essential for understanding these concepts. The triple point is a single point, the boiling point is a line (the liquid-vapor coexistence curve), and this line terminates at the critical point, where the latent heat of vaporization vanishes.

Answer 1

The Correct Option is B

Step 1: Analyze each statement. - A. Specific heat of saturated water vapour at 100\(^{\circ}\)C is negative: Saturated vapor is vapor in equilibrium with its liquid phase. If you add heat to saturated steam, it will superheat, and to keep it saturated, you must compress it. This compression work increases the temperature more than the added heat, so to maintain a constant temperature, you would actually have to remove heat. Alternatively, if you expand saturated steam adiabatically (no heat exchange), it cools and condenses. To prevent condensation and keep it saturated during expansion, you must add heat. The specific heat is defined per unit temperature change, and under certain processes for saturated steam, it can indeed be negative. So, A is correct. - B. There is only one triple point of a substance: The triple point is the unique combination of temperature and pressure at which the solid, liquid, and gas phases of a substance can all coexist in thermodynamic equilibrium. For a pure substance, this is a unique, fixed point. So, B is correct. - C. Boiling point of every liquid rises with increase in pressure: The boiling point is the temperature at which the vapor pressure of a liquid equals the surrounding pressure. For all liquids, vapor pressure increases with temperature. Therefore, to make the liquid boil at a higher temperature, you must increase the external pressure. So, C is correct. - D. Latent heat can not become zero: The latent heat of vaporization is the energy required to change a substance from liquid to gas at constant temperature and pressure. As you increase the temperature and pressure along the boiling curve, you approach the critical point. At the critical point, the distinction between liquid and gas phases disappears, and the latent heat of vaporization becomes zero. So, D is incorrect.
Step 2: Conclude the set of correct statements. Statements A, B, and C are correct.