If a substance undergoes a phase change without a rise in temperature, the energy involved is called?

• A. Specific Heat
• B. Latent Heat
• C. Sensible Heat
• D. Thermal Energy

Answer: (B)

The energy involved in a phase change at a constant temperature is latent heat. When a substance changes phase without warming or cooling, the added or removed energy goes into changing the arrangement of molecules—overcoming intermolecular forces—rather than increasing kinetic energy, which would raise temperature. Hence the heat is “hidden” in the phase transition, not as a temperature rise.

Latent heat appears in two common forms: fusion (solid to liquid) and vaporization (liquid to gas). For example, melting ice at 0°C requires about 334 kJ of energy per kilogram to break the rigid ice lattice and form liquid water, while boiling water at 100°C requires about 2260 kJ/kg to overcome intermolecular forces and produce steam. If the process releases energy, such as freezing or condensation, latent heat is released to the surroundings.

Sensible heat, by contrast, is the heat that changes the temperature of a substance without changing its phase. Thermal energy is the total energy content of the substance, not specifically the energy involved in a phase change. Specific heat relates to how much energy is needed to raise temperature, which is relevant to sensible heating.