How many BTUs are needed to convert one pound of water to vapor?

How many BTUs are needed to convert one pound of water to vapor?

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

Answer: (D)

When water changes from a liquid to a gas at its boiling point, the needed energy is the latent heat of vaporization. For water, this value is about 2256 kilojoules per kilogram, which converts to roughly 970 BTUs per pound. That energy goes into breaking intermolecular attractions rather than raising temperature, so one pound of water at 212°F (100°C) requires about 970 BTUs to become steam. If the water isn’t already at boiling, you’d first need additional sensible heat to lift it to the boiling temperature, and that extra energy adds to the total. The other numbers correspond to different processes (for example, melting ice requires about 144 BTU per pound), not vaporization.

When water changes from a liquid to a gas at its boiling point, the needed energy is the latent heat of vaporization. For water, this value is about 2256 kilojoules per kilogram, which converts to roughly 970 BTUs per pound. That energy goes into breaking intermolecular attractions rather than raising temperature, so one pound of water at 212°F (100°C) requires about 970 BTUs to become steam. If the water isn’t already at boiling, you’d first need additional sensible heat to lift it to the boiling temperature, and that extra energy adds to the total. The other numbers correspond to different processes (for example, melting ice requires about 144 BTU per pound), not vaporization.