Question

why are the latent heat of fusion and the latent heat of vaporization said to be hidden?
multiple choice
molecular potential energy is increased as water vapor is condensed or water freezes.
heat is absorbed or released, but a temperature change does not take place, so they are hidden.
molecular potential energy is decreased as energy is absorbed, which is hidden.

Explanation:

Latent heat of fusion and vaporization involve heat transfer during phase changes (e.g., melting/freezing, boiling/condensing). During these phase - change processes, heat is absorbed (e.g., melting, boiling) or released (e.g., freezing, condensing). However, the temperature of the substance remains constant during the phase change. Since the heat transfer is not accompanied by a temperature change (which is a common way we "observe" heat effects), it is said to be "hidden".

• For the first option: When water vapor condenses (releases heat) or water freezes (releases heat), molecular potential energy decreases (not increases). So this option is incorrect.
• For the third option: When energy is absorbed (e.g., during melting or vaporization), molecular potential energy increases. So this option is incorrect.

Answer:

Heat is absorbed or released, but a temperature change does not take place, so they are "hidden."