Advanced Placement (AP) Chemistry Practice Exam 2025 - Free AP Chemistry Practice Questions and Study Guide

The heating curve of a substance effectively illustrates temperature changes during phase transitions over time. As a substance is heated, its temperature rises until it reaches a phase transition point, such as melting or boiling. At these transition points, the temperature remains constant despite the continued addition of heat, as the energy supplied is used to change the state of the substance rather than increase its temperature.

For instance, during the melting phase, energy is absorbed to break the intermolecular forces in a solid, allowing it to become a liquid. Similarly, during boiling, the energy input helps to convert the liquid into a gas. The graph visually captures these changes, clearly showing the segments where the temperature increases and the plateaus where phase transitions occur. This characteristic representation is crucial for understanding the thermodynamic behavior of substances as they undergo heating.

When comparing this to the other options, the heating curve does not illustrate changes in pressure (the first choice) or relate volume to temperature at constant pressure (the third choice), as it specifically focuses on temperature without considering pressure variables. Additionally, the change in density as a substance is heated (the fourth choice) can happen, but it is not the central focus of a heating curve, which prioritizes changes in temperature during phase transitions.