AWH 6-6

Chapter 6, Water Vapor 6-6 6.7.1 Latent Heat Latent heat is the quantity of heat energy either released or absorbed by a unit mass of a substance when it undergoes a phase transition (change of state). Units are typically expressed in terms of joules per gram (J/g). Figure 6-4 illustrates the latent heat transactions that occur when water undergoes phase transition. Figure 6-4. Latent Heat Transactions When Water Undergoes Phase Transition Heat is exchanged between water and its environment during phase transition. Although the temperature of the environment changes in response, the temperature of the water undergoing the phase transition remains constant until the phase change is complete; that is, the available heat, latent heat, is involved exclusively in changing the phase of water and not in changing its temperature. There are six phase transitions, three of which are associated with the absorption of latent heat by water from the environment (melting, evaporation, and sublimation), and three of which are associated with the release of heat energy by water to the environment (freezing, condensation, and deposition). Melting is the phase transition by which a solid is changed to a liquid. During melting, water absorbs 334 J/g due to the latent heat of fusion. Freezing, the reverse process, releases 334 J/g back to the environment. Evaporation is the phase transition by which a liquid is changed to a vapor. During evaporation, water absorbs 2,501 J/g due to the latent heat of vaporization. Condensation, the reverse process, releases 2,501 J/g back to the environment. Sublimation is the phase transition by which a solid is changed to a vapor. During sublimation, water absorbs 2,834 J/g due to the latent heat of sublimation. Deposition, the reverse process, releases 2,834 J/g back to the environment. Table 6-1 lists the latent heat exchanges of water.