Which of the following statements correctly describes a heating or cooling curve for a pure substance?

• A. The temperature increases continuously, even during phase changes.
• B. During a phase change, the temperature remains constant while heat is added or removed.
• C. Heating and cooling curves only apply to mixtures, not pure substances.
• D. No energy is required for a substance to change phase.

Answer: (B)

Verified step by step guidance

1. Understand that a heating or cooling curve represents how the temperature of a pure substance changes as heat is added or removed.
2. Recall that during a phase change (such as melting or boiling), the temperature of a pure substance remains constant because the added or removed heat is used to change the phase, not to change the temperature.
3. Recognize that the statement 'The temperature increases continuously, even during phase changes' is incorrect because temperature plateaus occur during phase changes.
4. Know that heating and cooling curves apply to pure substances as well as mixtures, but the behavior during phase changes is distinct for pure substances due to their fixed melting and boiling points.
5. Remember that energy is required for a substance to change phase; this energy is called latent heat, which is absorbed or released without changing temperature.