A substance is heated at constant pressure. Which of the following temperature vs. time charts correctly represents the heating of a pure substance as it undergoes a phase change?

• A. A chart showing temperature increasing with time, but with horizontal plateaus during phase changes.
• B. A chart showing temperature remaining constant throughout the entire heating process.
• C. A chart showing a continuous increase in temperature with time, with no plateaus.
• D. A chart showing temperature decreasing with time as heat is added.

Answer: (A)

Verified step by step guidance

1. Understand that when a pure substance is heated at constant pressure, its temperature changes depending on whether it is undergoing a phase change or not.
2. Recall that during a phase change (such as melting or boiling), the temperature remains constant even though heat is continuously added. This is because the heat energy is used to break intermolecular forces rather than increase kinetic energy.
3. Recognize that before and after the phase change, the temperature of the substance increases as heat is added, which corresponds to an upward slope on a temperature vs. time graph.
4. Therefore, the correct temperature vs. time graph for heating a pure substance at constant pressure will show increasing temperature segments interrupted by horizontal plateaus where the temperature remains constant during phase changes.
5. Eliminate options that show temperature constant throughout (no increase), continuous increase without plateaus, or temperature decreasing with added heat, as these do not represent the physical behavior of a pure substance during heating and phase changes.