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All changes in state are between a condensed state and a less condensed state. For example the boiling of a liquid converts a condensed state (the liquid) with particles close together to a dispersed state (the gas). |
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Changes in state are accompanied by a change in potential energy. This should be clear because the change in state causes a change in the positions of particles. Remember potential energy is energy resulting from position. There are definite transition temperatures at definite pressures. The temperatures are pressure sensitive. Pressure is more important when there are big differences between the volumes of the initial and final states. |
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The common phase or state changes are listed in this table. |
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Condensed state |
process |
Less condensed state |
Heat effect |
Name for heat effect |
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Solid |
melting----> |
Liquid |
endothermic |
Heat of fusion |
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Liquid |
boiling -----> |
Gas |
endothermic |
Heat of vaporization |
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Solid |
sublimation---> |
Gas |
endothermic |
Heat of sublimation |
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Gas |
condensation--> |
Liquid |
exothermic |
Heat of condensation |
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Gas |
condensation--> |
Solid |
exothermic |
Heat of condensation |
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Liquid |
freezing ------> |
Solid |
exothermic |
Heat of crystallization |
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Note that the changes are endothermic when the system goes from a condensed state to a less condensed state. This results from the fact that energy is needed to "pry" apart a condensed state. The particles are attracted to one another in the condensed state by intermolecular forces. The stronger the forces the bigger the energy change. The reverse process has the opposite sign. |
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