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Figure a shows conversion from solid to liquid. In the solid substance, molecules are seen as small circles arranged in a grid. They are connected to one another through springs, forming a lattice structure. Each molecule has a small arrow originating from it. These arrows point in different directions. The length of the arrow forms the radius of a circle labeled limits of motion. In the liquid, the molecules are further apart from each other than in the solid. Arrows from the molecules indicate that they move in any direction. An arrow from the solid to the liquid is labeled energy input, melt. An arrow from the liquid to the solid is labeled energy output, freeze. Figure b shows the liquid and gas, where the molecules are even further apart than the liquid. In the gas, too, they move in any direction. An arrow from the liquid to the gas is labeled energy input evaporate. An arrow from gas to liquid is labeled energy output, condense.
(a) Energy is required to partially overcome the attractive forces (modeled as springs) between molecules in a solid to form a liquid. That same energy must be removed from the liquid for freezing to take place. (b) Molecules become separated by large distances when going from liquid to vapor, requiring significant energy to completely overcome molecular attraction. The same energy must be removed from the vapor for condensation to take place.

[link] lists representative values of L f and L v in kJ/kg, together with melting and boiling points. Note that in general, L v > L f . The table shows that the amounts of energy involved in phase changes can easily be comparable to or greater than those involved in temperature changes, as [link] and the accompanying discussion also showed.

[1] Values quoted at the normal melting and boiling temperatures at standard atmospheric pressure ( 1 atm ). [2] Helium has no solid phase at atmospheric pressure. The melting point given is at a pressure of 2.5 MPa. [3] At 37.0 ° C (body temperature), the heat of vaporization L v for water is 2430 kJ/kg or 580 kcal/kg. [4] At 37.0 ° C (body temperature), the heat of vaporization, L v for water is 2430 kJ/kg or 580 kcal/kg.
Heats of fusion and vaporization [1]
L f L v
Substance Melting Point ( °C ) kJ/kg kcal/kg Boiling Point ( °C ) kJ/kg kcal/kg
Helium [2] 272.2   ( 0.95  K ) 5.23 1.25 −268.9 ( 4.2 K ) 20.9 4.99
Hydrogen −259.3 ( 13.9 K ) 58.6 14.0 −252.9 ( 20.2 K ) 452 108
Nitrogen −210.0 ( 63.2 K ) 25.5 6.09 −195.8 ( 77.4 K ) 201 48.0
Oxygen −218.8 ( 54.4 K ) 13.8 3.30 −183.0 ( 90.2 K ) 213 50.9
Ethanol –114 104 24.9 78.3 854 204
Ammonia –75 332 79.3 –33.4 1370 327
Mercury –38.9 11.8 2.82 357 272 65.0
Water 0.00 334 79.8 100.0 2256 [3] 539 [4]
Sulfur 119 38.1 9.10 444.6 326 77.9
Lead 327 24.5 5.85 1750 871 208
Antimony 631 165 39.4 1440 561 134
Aluminum 660 380 90 2450 11400 2720
Silver 961 88.3 21.1 2193 2336 558
Gold 1063 64.5 15.4 2660 1578 377
Copper 1083 134 32.0 2595 5069 1211
Uranium 1133 84 20 3900 1900 454
Tungsten 3410 184 44 5900 4810 1150

Phase changes can have a strong stabilizing effect on temperatures that are not near the melting and boiling points, since evaporation and condensation occur even at temperatures below the boiling point. For example, air temperatures in humid climates rarely go above approximately 38.0 ° C because most heat transfer goes into evaporating water into the air. Similarly, temperatures in humid weather rarely fall below the dew point—the temperature where condensation occurs given the concentration of water vapor in the air—because so much heat is released when water vapor condenses.

More energy is required to evaporate water below the boiling point than at the boiling point, because the kinetic energy of water molecules at temperatures below 100 ° C is less than that at 100 ° C , so less energy is available from random thermal motions. For example, at body temperature, evaporation of sweat from the skin requires a heat input of 2428 kJ/kg, which is about 10% higher than the latent heat of vaporization at 100 ° C . This heat comes from the skin, and this evaporative cooling effect of sweating helps reduce the body temperature in hot weather. However, high humidity inhibits evaporation, so that body temperature might rise, while unevaporated sweat might be left on your brow.

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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