Iodine crystals have a significant vapor pressure at room...

Iodine crystals have a significant vapor pressure at room temperature and form a rich purple vapor when allowed to stand in a sealed container (Figure 15.22). A sealed container is filled with $10.0$ grams of solid iodine crystals and allowed to reach equilibrium between its solid and vapor phases at $25^{\circ} \mathrm{C}$. How will the vapor color change if (a) the container is heated to $90^{\circ} \mathrm{C} ?$ (b) the container is cooled to $-5^{\circ} \mathrm{C} ?$ (c) an additional $10.0$ grams of iodine crystals are added to the container?

Iodine crystals have a significant vapor pressure at room temperature and form a rich purple vapor when allowed to stand in a sealed container (Figure 15.22). A sealed container is filled with $10.0$ grams of solid iodine crystals and allowed to reach equilibrium between its solid and vapor phases at $25^{\circ} \mathrm{C}$. How will the vapor color change if
(a) the container is heated to $90^{\circ} \mathrm{C} ?$
(b) the container is cooled to $-5^{\circ} \mathrm{C} ?$
(c) an additional $10.0$ grams of iodine crystals are added to the container?

Key Concepts

Addition of Excess Solid

In a system at equilibrium between a solid and its vapor, adding more solid does not change the vapor pressure, as it is a property solely of the substance at that particular temperature. The equilibrium remains maintained because any additional solid will simply dissolve into the vapor phase until the established vapor pressure is reached again, leaving the concentration and appearance of the vapor unchanged.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. It reflects the tendency of molecules to escape from the solid or liquid phase into the vapor phase and generally increases with temperature as more molecules gain sufficient energy to transition into the gas phase.

Phase Equilibrium

Phase equilibrium describes the state in which two or more phases of a substance coexist at a constant temperature and pressure without any net change in the amounts of each phase. In a closed system containing a solid and its vapor, the system reaches an equilibrium determined solely by temperature, meaning that the vapor pressure is fixed when the system is at equilibrium.

Temperature Dependence of Equilibrium Vapor Pressure

The equilibrium vapor pressure is highly dependent on temperature; when the temperature increases, the vapor pressure rises because more molecules have sufficient energy to transition from the solid phase into the vapor phase. Conversely, lowering the temperature decreases the number of molecules escaping into the vapor phase, thereby reducing the vapor pressure. This variation in vapor pressure directly affects the concentration of the vapor, which can be observed as a change in the intensity of the vapor’s color.

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