Which formula computes the energy required to vaporize a mass m of water at 100°C?

• A. Q = m × Lf
• B. Q = m × Lv
• C. Q = m × Cp × ΔT
• D. Q = m × (Lv - Lf)

Answer: (B)

At the boiling point, the energy added to water goes into latent heat of vaporization—the energy needed to change liquid water into vapor without a change in temperature. The amount of energy required per unit mass is Lv, so the total energy is Q = m × Lv. This is why the correct formula uses Lv times the mass.

The other expressions don’t fit the situation: Lf is the latent heat of fusion (melting ice to water), which isn’t involved when water at 100°C vaporizes; Cp × ΔT is the sensible heat for changing temperature without a phase change (not applicable here since the temperature stays at 100°C during vaporization); and taking Lv − Lf isn’t the standard way to calculate vaporization energy.