A cooling curve has two flat lines, or plateaus. What does the plateau at the higher temperature represent?

Answer: The plateau at the higher temperature in a cooling curve represents the phase change of a substance from liquid to solid, which is also known as the process of solidification or freezing. During this phase change, the temperature of the substance remains constant despite the continuous removal of heat. This is because the energy being removed is used to overcome the forces of attraction between the liquid particles, allowing them to arrange into a more rigid, fixed structure of a solid. This phase change occurs at a characteristic temperature for each pure substance, which is known as its freezing point or melting point (the terms are interchangeable depending on the direction of the phase change). Accordingly, the plateau on a cooling curve indicates that the substance is undergoing a change in state rather than a change in temperature.