Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

Problem Strategy The cooling effect as CCl2F2 liquid evaporates is used to freeze water at 0_C to ice. You first need to obtain the quantity of heat removed from the water to freeze it (using the heat of fusion of ice as a conversion factor, converting moles of H2O to the heat required). Then you use this quantity of heat to determine the amount of CCl2F2 to evaporate (now using the heat of vaporization of CCl2F2 to convert this quantity of heat to the amount of CCl2F2 that must be vaporized).

Solution The heat that must be removed to freeze 525 g of water at 0_C is

11.2 Phase Transitions

(continued)

■ See Problems 11.43, 11.44, 11.45, and 11.46. The minus sign indicates that heat energy is taken away from the water. Consequently, the vaporization of dichlorodifluoromethane absorbs 175 kJ of heat. The mass of CCl2F2 that must be vaporized to absorb this quantity of heat is