In the heating curve, heat is applied to a solid substance at a constant rate. What accounts for the fact that segment CD is longer than segment AB?

Assuming a constant rate of heat flow, Segment AB calculates the amount of time needed to fully melt a solid into a liquid (or freeze a liquid into a solid). The longer the melting process, the more energy is needed.

The concept is the same for segment CD, but it deals with the amount of time needed to evaporate a liquid into a gas (or condense a gas into a liquid); in this case, we need to eliminate the majority of intermolecular forces and allow the molecules to move as gases.

Therefore, it takes more time and energy to fully evaporate or condense a substance than it does to fully freeze or melt it.

Segment CD is longer than segment AB in the heating curve because during segment CD, the substance is undergoing a phase change (from liquid to gas) while absorbing heat energy, which requires more energy input compared to segment AB, where the substance is only increasing in temperature.

Segment CD is longer than segment AB in the heating curve because during segment CD, the solid substance is undergoing a phase change from a liquid to a gas (specifically, from liquid to vapor). This phase change requires the input of additional heat energy to overcome the intermolecular forces holding the particles together in the liquid phase. As a result, more heat energy is required to break these intermolecular bonds and convert the substance from liquid to vapor, leading to a longer duration of heating compared to segment AB, where the substance is only undergoing an increase in temperature while remaining in the solid phase.