1FPD_CBL_TI83.WPD09/17/02Figure 1 Hypothetical Cooling CurveExperiment2Colligative Properties of SolutionsIntroduction:Consider a hypothetical cooling curve which describes the changes in phase of a pure substance in whichheat is lost at a steady rate at constant pressure (see figure 1).Note that the gas, liquid, and solid have differentheat capacities and cool at different rates.This is responsible for the different negativeslopes for cooling of each phase in the abovecurve. Kinetic energy is the energy of motionand is directly proportional to the absolutetemperature. At high temperatures, a stablecompound will have enough kinetic energy toexist as independent molecules in the gaseousstate. As heat is lost, the temperature andaverage kinetic energy of the gas decline. Apoint is reached at which intermolecularattractive forces become competitive with thekinetic energy and the molecules begin toassociate in the liquid phase.Here the molecules maintain continuous contact but still retain enough energy to migrate randomlythroughout the system. As gas molecules continue to condense to liquid, potential energy is lost as heatand the temperature of the system remains the same until all of the gas has liquefied (this is the boilingtemperature of the liquid). Now with only liquid present, the liquid cools and the temperature drops untilthe average kinetic energy becomes low enough for the intermolecular forces to lock the molecules inplace within a solid crystal lattice; the molecules are no longer free to wander. As solidification(freezing) continues, the temperature of the system remains constant as potential energy converts to heatand is released (this the freezing or melting temperature of the liquid). When conversion to solid iscomplete, the temperature of the system then declines as the solid itself cools. The molecules within thelattice vibrate in place, and as the temperature falls a minimum in vibrational energy is approached(absolute temperature, 0 K , or -273/C).
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