Class21 - Phase Equilibrium When a substance changes phase...

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2/24/12 Oregon State University PH 202, Lecture 21 1 Phase Equilibrium When a substance changes phase due to heat gain (or loss), not all of its molecules change phase simultaneously. The first Joules of heat to arrive (or leave) let some of the molecules escape (or re-bond); others have to wait their turn until more thermal (kinetic) energy arrives (or leaves). But the transitions don’t proceed in just one direction . As water boils, for example, most phase-changing particles are transitioning from liquid to gas. But some particles are going the other way— condensing—instead. Why? Keep in mind that temperature is a statistical quantity—an indication of average particle kinetic energy. At any one time, a given particle may have more or less than the kinetic energy required to maintain its presence in a particular phase.
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2/24/12 Oregon State University PH 202, Lecture 21 2 Vapor Pressure If you place a liquid substance in an evacuated chamber, initially it will boil vigorously; far more liquid-to-gas transitions occur than vice versa. But eventually, enough gas fills the chamber space above the liquid so that its pressure is sufficient to cause gas-to-liquid
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Class21 - Phase Equilibrium When a substance changes phase...

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