---
title: "A student evaluates a pure molecular substance for use in a thermal-storage device. The initially solid sample is heated at constant pressure, and the student records its temperature as energy is added. The resulting heating curve is shown.  Which statement correctly describes the primary changes in the particles’ average kinetic energy and potential energy during regions III and IV of the graph?"
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url: "https://nerd-notes.com/ubq/119432/"
date_modified: "2026-08-19T12:40:18+00:00"
---

# A student evaluates a pure molecular substance for use in a thermal-storage device. The initially solid sample is heated at constant pressure, and the student records its temperature as energy is added. The resulting heating curve is shown.

Which statement correctly describes the primary changes in the particles’ average kinetic energy and potential energy during regions III and IV of the graph?

A student evaluates a pure molecular substance for use in a thermal-storage device. The initially solid sample is heated at constant pressure, and the student records its temperature as energy is added. The resulting heating curve is shown.

Which statement correctly describes the primary changes in the particles’ average kinetic energy and potential energy during regions III and IV of the graph?

![Draw a grayscale heating-curve graph with a bare white plotting area and no gridlines. The horizontal axis is labeled “Energy added to sample, q”; the vertical axis is labeled “Temperature.” A single solid polyline begins near the lower left and consists of exactly five connected segments. The first segment slopes upward moderately and is marked I. The second segment is horizontal and is marked II. The third segment slopes upward more gently than the first and is marked III. The fourth segment is horizontal, longer than the second, and is marked IV. The fifth segment slopes upward steeply and is marked V, ending near the upper right. Place each Roman numeral centered just above its segment, without numerical tick values or a legend. Use thin black axes and one medium-weight black curve. No other curves, labels, text, symbols, or annotations appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1787143218-QOAuhB.jpg)

- **A.** In region III, potential energy increases while average kinetic energy remains constant; in region IV, average kinetic energy increases while potential energy remains approximately constant, because a sloped segment represents a phase change and a plateau represents warming within one phase.
- **B.** In both regions III and IV, average kinetic energy increases while potential energy remains approximately constant, because continued energy transfer always increases particle speed.
- **C.** In region III, average kinetic energy increases while potential energy remains approximately constant; in region IV, average kinetic energy remains constant while potential energy decreases, because separating the particles lowers their intermolecular potential energy.
- **D.** In region III, average kinetic energy increases while potential energy remains approximately constant; in region IV, average kinetic energy remains constant while potential energy increases, because the added energy overcomes intermolecular attractions during vaporization.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119432/*