Latent Heat and Freezing and Boiling Points


A calorie is the amount of heat it takes to raise the temperature of 1 gram (0.001 liters) of pure water 1 degree C at sea level. It takes 100 calories to heat 1 g. water from 0˚, the freezing point of water, to 100˚ C, the boiling point. However, 540 calories of energy are required to convert that 1 g of water at 100˚ C to 1 g of water vapor at 100˚ C. This is called the latent heat of vaporization. On the other hand, you would have to remove 80 calories from 1 g of pure water at the freezing point, 0˚ C, to convert it to 1 g of ice at 0˚ C. This is called the latent heat of fusion.

Interestingly, the latent heat and freezing and boiling points are controlled by the way water molecules interact with one another. Because molecules acquire more energy as they warm, the association of water molecules as clusters begins to break up as heat is added. In other words, the energy is absorbed by the fluid and molecules begin to dissociate from one another. Considerable energy is required to break up the water molecule clusters, thus there is relatively little temperature change of the fluid for a given amount of heating (this is the heat capacity measure), and, even at the boiling point, it takes far more energy to liberate water molecules as a vapor (parting them from one another). On the other hand, when energy is removed from water during cooling the molecules of water begin to coalesce into clusters and this process adds energy to the mix, thus offsetting the cooling somewhat.

Graph shows latent heat of evaporation is 540 cal/gm and the latent heat of fusion is 80 cal/gm
Figure 2: Graph shows latent heat of evaporation and latent heat of fusion
Click here for a text description
Phase diagram of water. The temperature on the y-axis and heat input on the x-axis. Below 0C is ice, 0C to 100C is liquid water and above 100C is water vapor. Starting below 0C as heat is added the line rises steeply to 0C where it temporarily levels out. This is the latent heat of fusion or melting which is roughly 80cal/gram. The line is flat because all energy is going to the phase change and not raising the temperature. After the water has melted the line rises steeply as heat is added. This is the liquid water phase. At 100C the line once again levels off as the water is now boiling and the heat is going into the latent heat of evaporation which is roughly 540cal/gram. Once the water is above 100C it is now a water vapor.
Source: Mike Arthur and Demian Saffer