Latent heat from melting plateau energy input

Steps:

• From the graph, identify the time interval Δt for the flat melting plateau where temperature is constant.
• Calculate total thermal energy E supplied during melting: E = power × Δt = (1200 J/min) × Δt (in min).
• Latent heat of fusion L_f = E / mass = [(1200 × Δt) J] / 200 g.
• Using graph's Δt = 40 min, L_f = (1200 × 40) / 200 = 48,000 / 200 = 240 J/g.

Why B is correct:

• Matches L_f = 240 J/g, the energy per gram needed to change solid wax to liquid at constant temperature (definition of latent heat of fusion).

Why the others are wrong:

• A: 180 J/g underestimates energy, implying shorter Δt ≈ 30 min than graph shows.
• C: 480 J/g overestimates, implying Δt ≈ 80 min.
• D: 600 J/g greatly overestimates, implying Δt ≈ 100 min.

Final answer: B