Had we begun with the solid phase and warmed up to

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Had we begun with the solid phase and warmed up to the gas phase, the ΔH associated with the heat transfer would have been positive since the warming process is endothermic.
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Sublimation Deposition H 2 O ( s ) H 2 O ( g ) Molar heat of sublimation ( H sub ) is the energy required to sublime 1 mole of a solid. H sub = H fus + H vap ( Hess’s Law)
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Some solids that sublime: CO 2 (dry ice), iodine, naphthalene (moth balls), solid room deodorizers, etc… What causes a solid to sublime rather than melt? It is because the combination of intermolecular forces in the solid and atmospheric pressure is not great enough to keep particles near one another enough when they leave the solid state (i.e., they don’t enter the liquid phase). For example, solid CO 2 has such weak intermolecular forces that, at atmospheric pressure, when the CO 2 is warmed, the molecules directly separate to gaseous distances, and bypass the liquid phase. Only at an increased external pressure (around 5 atm) is there enough pressure to keep the particles close enough together to form liquid CO 2 .
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A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water Phase-transition Curves: Solid-vapor Liquid-vapor Solid-liquid Triple Point Critical Point
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The phase diagram for water is different than almost every other substance in that it has a negative slope for the solid-liquid transition curve. Solid water is less dense than liquid water, due to solid water having a greater volume than liquid water. Water expands upon freezing due to the unique crystalline structure of ice. An increase in external pressure always favors the phase that occupies the less volume (increase in density). For water, this phase is liquid. Hence the negative slope of the solid-liquid line: ↑P ↓FP of H 2 O (i.e., if you apply pressure to ice, it will melt) (This is the opposite of what you find with almost all other substances, where an increase in P an increase in FP.)
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Phase Diagram of Carbon Dioxide At 1 atm CO 2 ( s ) CO 2 ( g )
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Effect of Increase in Pressure on the Melting Point of Ice and the Boiling Point of Water
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The critical temperature ( T c ) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure ( P c ) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature.
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The Critical Phenomenon of SF 6 T < T c T > T c T ~ T c T < T c
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What exists beyond the critical point on a phase diagram? A supercritical fluid (SCF): A SCF expands and contracts like a gas, but has solvent properties like a liquid. Supercritical CO 2 is used to extract caffeine from coffee beans, nicotine from tobacco, and fats from potato/corn chips. Supercritical H 2 O is being studied for its potential to dissolve nonpolar substances like organic toxins (PCBs, etc.) from water.
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Chemistry In Action: Liquid Crystals
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