During the phase change from solid to liquid at constant temperature, the heat absorbed for mass m is given by Q = m L. Which expression correctly represents Q?

During the phase change from solid to liquid at constant temperature, the heat absorbed for mass m is given by Q = m L. Which expression correctly represents Q?

• A. Q = m c ΔT
• B. Q = m L
• C. Q = L/m
• D. Q = m^2 L

Answer: (B)

During melting, the temperature stays constant while the solid changes to a liquid, so all the added energy goes into changing the phase, not raising the temperature. The energy required per unit mass to melt is the latent heat of fusion, L, and the total heat absorbed is Q = mL. For example, melting 2 kg of ice at 0°C with a latent heat of fusion of 334 kJ/kg requires Q = 2 × 334 kJ = 668 kJ. The formula Q = m c ΔT would apply only if the temperature were changing, and the other forms with L/m or m^2L don’t have the correct units or meaning for this process.

During melting, the temperature stays constant while the solid changes to a liquid, so all the added energy goes into changing the phase, not raising the temperature. The energy required per unit mass to melt is the latent heat of fusion, L, and the total heat absorbed is Q = mL. For example, melting 2 kg of ice at 0°C with a latent heat of fusion of 334 kJ/kg requires Q = 2 × 334 kJ = 668 kJ. The formula Q = m c ΔT would apply only if the temperature were changing, and the other forms with L/m or m^2L don’t have the correct units or meaning for this process.