What is the energy requirement for a total phase change of liquid water to steam at 212°F?

The energy requirement for the total phase change of liquid water to steam at 212°F, which is the boiling point of water at atmospheric pressure, is 970 BTUs. This value represents the latent heat of vaporization, which is the amount of energy needed to change a substance from a liquid to a gas without changing its temperature.

When water reaches its boiling point, it requires a significant amount of energy to break the hydrogen bonds between water molecules in liquid form so that they can move freely as steam. This energy, 970 BTUs per pound of water, accounts for the phase change itself rather than raising the temperature since the temperature remains constant during the process until all of the liquid water has turned to steam.

Understanding this concept is crucial for applications in heating systems, boiler operations, and other processes involving phase changes of water. The other choices provided do not reflect the correct amount of energy needed for complete vaporization at this temperature, highlighting the importance of recognizing the specific value for the boiling phase change of water.