Which energy value represents vaporizing one pound of water into vapor?

Which energy value represents vaporizing one pound of water into vapor?

• A. 144 BTUs
• B. 10 BTUs
• C. 80 BTUs
• D. 970 BTUs

Answer: (D)

Vaporizing water is a phase-change at the boiling point, so the energy asked for is the latent heat of vaporization. For water, this value is about 970 BTU per pound, representing the energy required to overcome molecular forces and turn liquid water at its boiling point into steam without raising the temperature. That’s why 970 BTUs per pound is the correct figure. For contrast, melting ice into water requires about 144 BTU per pound (latent heat of fusion), which is a different phase change, not vaporization. The other listed numbers don’t correspond to the energy needed to vaporize water.

Vaporizing water is a phase-change at the boiling point, so the energy asked for is the latent heat of vaporization. For water, this value is about 970 BTU per pound, representing the energy required to overcome molecular forces and turn liquid water at its boiling point into steam without raising the temperature. That’s why 970 BTUs per pound is the correct figure. For contrast, melting ice into water requires about 144 BTU per pound (latent heat of fusion), which is a different phase change, not vaporization. The other listed numbers don’t correspond to the energy needed to vaporize water.