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Section 4 -Phase Equilibrium

Section 4 -Phase Equilibrium. Two-Phase Systems. A system is a set of components that are being studied. Within a system, a phase is a region that has the same composition and properties throughout. A glass of water and ice cubes is a system that has two phases. This system

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Section 4 -Phase Equilibrium

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  1. Section 4 -Phase Equilibrium Two-Phase Systems A system is a set of components that are being studied. Within a system, a phase is a region that has the same composition and properties throughout. A glass of water and ice cubes is a system that has two phases. This system has a solid phase and a liquid phase. However, the two phases have the same chemical composition. What makes the two phases in ice water different from each other is that they are different states of the same substance, water.

  2. Equilibrium Involves Constant Interchange of Particles A dynamic equilibrium exists when particles are constantly moving between two or more phases yet no net change in the amount of substance in either phase takes place. Molecules are escaping from the solid phase into a gas at the same rate that other molecules are returning to the solid from the gas phase. That is, the rate of sublimation equals the rate of deposition.

  3. Vapor Pressure Increases with Temperature A closed container of water is a two-phase system in which molecules of water are in a gas phase in the space above the liquid phase. Moving randomly above the liquid, some of these molecules strike the walls and some go back into the liquid, as shown below.An equilibrium, in which the rate of evaporation equals the rate of condensation, is soon created. The molecules in the gas exert pressure when they strike the walls of the container. The pressure exerted by the molecules of a gas, or vapor, phase in equilibrium with a liquid is called the vapor pressure. You can define boiling point as the temperature at which the vapor pressure equals the external pressure. As the temperature of the water increases, the molecules have more kinetic energy, so more of them can escape into the gas phase. Thus, as temperature increases, the vapor pressure increases

  4. Vapor Pressures of Three Substances at Various Temperatures

  5. Phase Diagrams A phase diagram has three lines. One line is a vapor pressure curve for the liquid-gas equilibrium. A second line is for the liquid-solid equilibrium, and a third line is for the solid-gas equilibrium. All three lines meet at the triple point. The triple point is the only temperature and pressure at which three states of a substance can be in equilibrium.

  6. Phase Diagram for H2O Phase Diagram for CO2 Each pure substance has a unique phase diagram, although the general structure is the same.

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