Which equation represents the heat required for a phase change at constant temperature?

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

Answer: (D)

When a substance changes phase at a constant temperature, all the added energy goes into changing the arrangement of the molecules, not raising the temperature. This energy per unit mass is called latent heat, L. The total heat required for a mass m undergoing a phase change is Q = mL, with L depending on the specific phase transition (fusion or vaporization).

So the correct form captures that heat scales with mass and with the latent heat of the transition. The other forms describe heating with a temperature change, not a phase change, or are dimensionally inconsistent for latent heat.