Explain how the second heating experiment determines latent heat of vaporization.

Solution & explanation

At the stated pressure heat water already at its boiling point with known power. Measure evaporated mass Δm over interval Δt, preferably before and after rather than boiling the vessel dry. The effective energy for vaporization gives Lv = Q/Δm ≈ PΔt/Δm after corrections for losses. Constant temperature during boiling is the observed phase-change plateau.