The Dangers of Phase Changes

A heating curve can also be used to explain why hikers stranded by blizzards are warned not to consume snow in an effort to stay hydrated. When you drink cold water, your body expends energy to warm the water you consume to body temperature. If you consume snow, your body must first expend the energy necessary to melt the snow, and then to warm it. Because a phase change is involved, the amount of energy required to assimilate snow is much greater than the amount necessary to assimilate an equal mass of water—even if the water is ice-cold. This can contribute to hypothermia, a potentially dangerous drop in body temperature. 

Strategy For the purpose of following the sign conventions, we can designate the water as the system and the body as the surroundings, (a) Heat is transferred from hot water to the skin in a single 

Setup The required specific heats (s) are 4.184 J/g °C for water and 1.99 J/g °C for steam. (Assume that the specific heat values do not change over the range of temperatures in the problem.) From Table 12.6, the molar heat of vaporization of water is 40.79 kJ/mol, and from 

02 g/mol. Note The A//vap of water is the amount of heat required to vaporize a mole of water. 

In this problem, however, we want to know how much heat is deposited when water vapor condenses, so we must use the negative, 40.79 kJ/mol. 

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