Boiling of Water - continued.

Here’s what’s happening. 212 deg F is known as the “Saturation Temperature” for water. At this temperature it changes its phase from water to steam. But this does not happen in a straight forward manner.

To change its phase, water needs a shot of extra energy to break loose the molecules from the liquid phase into the vapor phase. One pound of water requires 970 BTUs of this “extra” energy. This extra energy is referred to as the “Latent Heat of Vaporization”.

Latent Heat of Vaporization is defined as the heat required by one pound of substance to change its phase from liquid to vapor.

As the water absorbs the latent heat, its temperature does not change. Gradually, some of the water begins to form steam. Still the temperature of the water does not change. As a matter of fact, at this stage, the temperature of the water, as well as that of the newly formed steam is 212 deg F.

Gradually water absorbs 970 BTUs and as a result we see complete conversion from liquid to the vapor phase. The vapor at this stage is referred to as “saturated vapor”.

Latent heat is not sensible heat because a thermometer cannot detect it. The word latent means, “hidden”.