Phase-change energetics: The heat required to vaporize 1 kg of a saturated liquid at its boiling temperature is called

Introduction / Context:
Heating a substance can raise its temperature (sensible heating) or change its phase at essentially constant temperature (latent heating). Understanding the distinction is essential for boiler design, distillation, refrigeration, and energy balances involving phase changes.

Given Data / Assumptions:

• Saturated liquid undergoes vaporization at its saturation temperature and pressure.
• Heat input occurs isothermally during the phase change.

Concept / Approach:
Latent heat of vaporization is the energy required to overcome intermolecular forces and create vapor at the saturation condition, without raising temperature. Sensible heat changes temperature within a single phase. Specific heat is a property that quantifies the sensible heat per unit mass per degree; it is not the latent heat itself. A fixed energy quantity like “1 kcal” is not a definition.

Step-by-Step Solution:

Verification / Alternative check:
Steam tables list h_fg as the difference between saturated vapor and saturated liquid enthalpies; that difference is precisely the latent heat of vaporization at the stated pressure/temperature.

Why Other Options Are Wrong:

• Specific heat: relates sensible heating within a phase.
• 1 kcal: a unit, not a property definition.
• Sensible heat: involves temperature change, not phase change.
• Compression work: mechanical, not the thermal quantity defined here.

Common Pitfalls:
Using specific heat to estimate boiling duty; always use latent heat h_fg for the isothermal phase-change portion of the duty.

Final Answer:
latent heat