Question 57

The Correct Answer is C. ### Explanation for why Option C is Correct The illustration SG-0001 represents a temperature/enthalpy diagram (heating/cooling curve) for water at atmospheric pressure, moving from ice to steam (or vice versa). This diagram features distinct regions representing phase changes (isothermal plateaus) and sensible heating/cooling (sloped sections). Region 4 is the upper plateau (flat section) corresponding to the transition between steam and water, occurring at $100^{\circ} \mathrm{C}$ (the boiling/condensation point at standard pressure). * **Process Direction:** The question specifies the substance is undergoing a **heat loss** (cooling process). * **Region 4 (Cooling):** When the system loses heat in this region, the steam is transitioning back into liquid water. * **Heat Type:** A phase change occurring at a constant temperature involves the latent heat. When steam turns into water, it releases (loses) its **latent heat of condensation**. Therefore, region 4, under heat loss conditions, represents **Steam losing its latent heat of condensation and changing its physical state to become water.** ### Explanation for why other options are incorrect **A) Water absorbing its latent heat of vaporization and changing its physical state to become steam.** * This describes the opposite process (vaporization), which involves **heat gain** (heating), not heat loss. While it occurs in Region 4, the direction of heat transfer is incorrect. **B) Water absorbing its specific heat and experiencing a rise in temperature.** * This describes sensible heating of liquid water (Region 3, with heat gain). Region 4 is a phase change (isothermal). **D) Steam losing its specific heat and experiencing a drop in temperature.** * This describes sensible cooling of steam (Region 5, with heat loss), where the temperature decreases below $100^{\circ} \mathrm{C}$ (if superheated steam is cooled, or if the saturated vapor is cooled once fully condensed). Region 4 is an isothermal phase change.