---
title: "A pure sample of a molecular substance is heated at a constant rate under a constant pressure of \\(1.0 \\text{ atm}\\). The heating curve for the sample is shown below.  Which of the following correctly compares the molar enthalpy of fusion, \\(\\Delta H_{\\text{fus}}\\), to the molar enthalpy of vaporization, \\(\\Delta H_{\\text{vap}}\\), of the substance and provides the correct particulate-level justification?"
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url: "https://nerd-notes.com/ubq/123835/"
date_modified: "2026-09-28T12:30:30+00:00"
---

# A pure sample of a molecular substance is heated at a constant rate under a constant pressure of \(1.0 \text{ atm}\). The heating curve for the sample is shown below.

Which of the following correctly compares the molar enthalpy of fusion, \(\Delta H_{\text{fus}}\), to the molar enthalpy of vaporization, \(\Delta H_{\text{vap}}\), of the substance and provides the correct particulate-level justification?

A pure sample of a molecular substance is heated at a constant rate under a constant pressure of \(1.0 \text{ atm}\). The heating curve for the sample is shown below.

Which of the following correctly compares the molar enthalpy of fusion, \(\Delta H_{\text{fus}}\), to the molar enthalpy of vaporization, \(\Delta H_{\text{vap}}\), of the substance and provides the correct particulate-level justification?

![A 2D Cartesian coordinate system with bare axes in grayscale. The horizontal axis is labeled 'Heat Added (kJ)' and the vertical axis is labeled 'Temperature (°C)'. A single solid black curve traces the heating curve of a pure substance from bottom-left to top-right through five connected linear segments: segment 1 rises with positive slope from (0, -20) to (2, 40); segment 2 is a flat horizontal plateau labeled 'Melting' extending from (2, 40) to (5, 40), spanning 3 kJ; segment 3 rises with positive slope from (5, 40) to (9, 120); segment 4 is a flat horizontal plateau labeled 'Vaporization' extending from (9, 120) to (18, 120), spanning 9 kJ; segment 5 rises with positive slope from (18, 120) to (20, 160). Dashed vertical projection lines connect the plateau endpoints to the horizontal axis. No other curves, labels, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1790598629-KCLoq9.jpg)

- **A.** \(\Delta H_{\text{fus}} > \Delta H_{\text{vap}}\), because the solid phase has a rigid, organized lattice that requires more energy to disrupt than separating molecules in the liquid phase.
- **B.** \(\Delta H_{\text{fus}} > \Delta H_{\text{vap}}\), because the rate of temperature increase in the liquid phase is greater than the rate of temperature increase in the gas phase.
- **C.** \(\Delta H_{\text{vap}} > \Delta H_{\text{fus}}\), because boiling requires overcoming essentially all intermolecular attractions between molecules, whereas melting only requires partially disrupting the intermolecular attractions in the solid lattice.
- **D.** \(\Delta H_{\text{vap}} > \Delta H_{\text{fus}}\), because covalent bonds within the molecules are broken during vaporization, whereas only intermolecular attractions are broken during melting.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123835/*