Page 221 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 7 Water and Atmospheric Moisture 185
Latent heat of melting
+80 calories (absorbed)
PHASE Water CHANGE 1 gram
+100 calories (absorbed)
–100 calories (released)
Latent heat of vaporization
+540 calories (absorbed)
Ice
1 gram 0°C
0°C
Water 1 gram 100°C
PHASE CHANGE
Water vapour 1 gram 100°C
–80 calories (released) Latent heat of freezing
–540 calories (released) Latent heat of condensation
(a) Latent heat absorbed or released in phase changes between ice and water and water vapour. To transform 1 g of ice at 0°C to 1 g of water vapour at 100°C requires 720 cal: 80 + 100 + 540.
▲Figure 7.5 Water’s heat-energy characteristics.
Animation
Water Phase Changes
same: both ice and water measure 0°C (Figure 7.5a). When the phase change is reversed and a gram of water freezes, latent heat is released rather than absorbed. The latent heat of melting and the latent heat of freezing are each 80 cal·g−1.
To raise the temperature of 1 g of water at 0°C to boiling at 100°C, we must add 100 calories (an increase of 1 C° for each calorie added). No phase change is in­ volved in this temperature gain.
Water Vapour, the Gas Phase Water vapour is an in­ visible and compressible gas in which each molecule moves independently of the others (Figure 7.2a). When the phase change from liquid to vapour is induced by boiling, it requires the addition of 540 cal for each gram, under normal sea­level pressure; this amount of energy is the latent heat of vaporization (Figure 7.5). When water vapour condenses to a liquid, each gram gives up its hidden 540 cal as the latent heat of condensation. We see water vapour in the atmosphere after condensation has occurred, in the form of clouds, fog, and steam. Perhaps you have felt the release of the latent heat of condensation on your skin from steam when you drained steamed vegetables or pasta or filled a hot teakettle.
In summary, the changing of 1 g of ice at 0°C to water and then to water vapour at 100°C—from a solid to a liquid to a gas—absorbs 720 cal (80 cal 1 100 cal 1 540 cal). Re­ versing the process, or changing the phase of 1 g of water vapour at 100°C to water and then to ice at 0°C, releases 720 cal into the surrounding environment. Go to the MasteringGeography website for an excellent animation illustrating these concepts.
The latent heat of sublimation absorbs 680 cal as a gram of ice transforms into vapour. Water vapour
(b) Latent heat exchange between water in a lake at 20°C and water vapour in the atmosphere, under typical conditions.
freezing directly to ice releases a comparable amount of energy.
Latent Heat Transfer
under Natural Conditions
In a lake or stream or in soil water, at 20°C, every gram of water that breaks away from the surface through evapo­ ration must absorb from the environment approximately 585 cal as the latent heat of evaporation (see Figure 7.5b). This is slightly more energy than would be required if the water were at a higher temperature (if the water is boil­ ing, 540 cal are required). You can feel this absorption of latent heat as evaporative cooling on your skin when it is wet. This latent heat exchange is the dominant cool­ ing process in Earth’s energy budget. (Remember from Chapter 4 that the latent heat of evaporation is the most
–585 calories (released)
Latent heat of condensation for 1 gram
of water
+585 calories (absorbed)
Latent heat of evaporation for 1 gram
of water
20°C