What is the amount of heat energy required to evaporate 1 pound of water?

What is the amount of heat energy required to evaporate 1 pound of water?

• A. 540 BTU
• B. 100 BTU
• C. 970 BTU
• D. 1500 BTU

Answer: (C)

Evaporating water requires latent heat—the energy needed to overcome the molecular forces holding the liquid together so it can become vapor without a rise in temperature. At standard atmospheric pressure, the amount of energy required to convert one pound of liquid water into steam is about 970 BTU. In other words, to fully vaporize 1 lb of water, you must absorb roughly 970 BTU, and the water stays at its boiling temperature during that phase change. This value is what HVAC calculations use when estimating steam generation or humidification needs. The other numbers don’t represent the energy needed for evaporation at 1 atm.

Evaporating water requires latent heat—the energy needed to overcome the molecular forces holding the liquid together so it can become vapor without a rise in temperature. At standard atmospheric pressure, the amount of energy required to convert one pound of liquid water into steam is about 970 BTU. In other words, to fully vaporize 1 lb of water, you must absorb roughly 970 BTU, and the water stays at its boiling temperature during that phase change.

This value is what HVAC calculations use when estimating steam generation or humidification needs. The other numbers don’t represent the energy needed for evaporation at 1 atm.