A cooling curve tracks a substance’s temperature over time as heat is continuously removed, typically illustrating a phase change, such as a liquid turning into a solid. The classification of this process as endothermic or exothermic depends entirely on the direction of energy flow during the phase transition.
Decoding the Terms: Exothermic and Endothermic
An exothermic process releases energy, typically heat, out of the system and into the environment. A common example is the burning of a candle or a chemical hot pack, where the temperature of the surroundings rises because of the energy output.
Conversely, an endothermic process absorbs energy, usually heat, into the system from its surroundings. This absorption causes the environment’s temperature to drop, such as when ice melts or a chemical cold pack is activated. Since a cooling curve involves a substance losing heat, the energy flow during the phase transition determines its classification.
Interpreting the Cooling Curve Graph
A cooling curve is a visual representation of how a substance’s temperature changes over a period of time. Temperature is plotted on the vertical Y-axis, while time is recorded on the horizontal X-axis. This graph is generated by placing a substance, often in its liquid state, into a cooler environment and measuring its temperature at regular intervals.
The resulting graph typically shows three distinct regions that illustrate the thermal history of the substance. Initially, the curve exhibits a steep, downward slope, which represents the liquid cooling down before any change of state begins. As the substance reaches its freezing or condensation temperature, the curve enters a flat section known as the plateau or “thermal arrest.” This plateau is the most significant feature of the curve, indicating a period where the temperature remains constant despite the continuous removal of heat. Once the entire substance has converted to a solid, the curve resumes a shallow, downward slope, showing the solid continuing to cool toward the temperature of the surroundings.
The Exothermic Nature of Phase Change
The phase change represented by the plateau on a cooling curve—specifically solidification (liquid to solid) or condensation (gas to liquid)—is an exothermic process. This classification is based on the requirement for molecules to release energy to form the more ordered structure of a solid or liquid. As a liquid cools, the molecules slow down, and to lock into the fixed, lower-energy positions of a solid crystal lattice, they must shed the excess energy that kept them separated and mobile in the liquid state. This stored internal energy is released into the surroundings as heat when the molecules form new bonds, which is a defining characteristic of an exothermic process. Therefore, the transformation from a higher-energy liquid state to a lower-energy solid state inherently involves a release of thermal energy.
Why Temperature Holds Steady During Cooling
The reason the temperature remains constant during the plateau, even though heat is being removed, is due to a phenomenon called Latent Heat. Latent heat refers to the “hidden” heat energy involved in a phase change that does not cause a change in temperature. During the solidification process, the energy being continuously lost to the surroundings is exactly balanced by the energy being released by the substance itself as it changes phase. This energy released during the transition is known as the latent heat of fusion. The heat removal by the environment is counteracted by the internal heat release from the solidifying molecules, keeping the average kinetic energy of the particles—which is what temperature measures—stable. Only after all the liquid has converted into a solid is there no more latent heat to release, allowing the temperature of the newly formed solid to finally begin dropping again.