Massachusetts 2nd Class Fireman's Practice Exam

How is latent heat best defined?

1. Heat associated with temperature change.

2. Heat absorbed or released during a phase change at constant temperature.

3. Heat that cannot be measured.

4. Heat required to heat a substance by 1 degree.

The correct answer is: Heat absorbed or released during a phase change at constant temperature.

Latent heat is best defined as the heat absorbed or released during a phase change at constant temperature. This concept is crucial in thermodynamics and is particularly relevant when discussing processes such as melting, freezing, vaporization, and condensation. During these phase changes, a substance can absorb or release a significant amount of energy without undergoing any change in temperature. For example, when ice melts into water, it absorbs heat from its surroundings, which is the latent heat of fusion. Even though energy is being added, the temperature of the ice-water mixture remains constant until all the ice has melted. Similarly, when water turns into steam, it absorbs heat (latent heat of vaporization), again without a change in temperature until the phase change is complete. This distinguishes latent heat from other forms of heat transfer that involve temperature changes, as it focuses on energy transitions that facilitate the change of state in a substance while keeping temperature constant. Understanding this allows firefighters and other professionals to grasp how different materials behave under heat stress and the importance of temperature control during fire suppression activities.