At the melting point and boiling point of a pure substance, what occurs as heat is added at constant pressure?

• A. The temperature increases rapidly and no phase change occurs.
• B. The temperature remains constant while the substance undergoes a phase change.
• C. The pressure decreases as the temperature increases.
• D. The substance decomposes into its constituent elements.

Answer: (B)

Verified step by step guidance

1. Understand that at the melting point and boiling point of a pure substance, the system is undergoing a phase change, such as solid to liquid (melting) or liquid to gas (boiling).
2. Recall that during a phase change at constant pressure, the temperature remains constant because the added heat is used to overcome intermolecular forces rather than increasing kinetic energy.
3. Recognize that the heat added during this process is called latent heat, which is the energy required to change the phase without changing temperature.
4. Note that since the temperature does not increase during the phase change, the correct description is that the temperature remains constant while the substance undergoes the phase change.
5. Understand that options involving temperature increase without phase change, pressure decrease, or decomposition are incorrect in the context of a pure substance at its melting or boiling point under constant pressure.