We´re asked to explain why sublimation is generally performed under reduced pressure.
Sublimation is the change in the physical state of a compound or element, from a solid to a gas.
Changes in the physical state of a compound or element depending on temperature and pressure are shown in a Phase diagram like the following one.
Here we see there are equilibrium lines between each phase where the material is in both phases. The red line corresponds to the interface of the solid phase with the gas phase.
Additionally, consider that vapor pressure is the partial pressure of gas molecules above the surface of a liquid or a solid.
Sublimations are generally performed under reduced pressure because:
a. the attractive intermolecular interactions between the molecules in the solid get weaker as the pressure is decreased.
b. solids do not have a vapor pressure at atmospheric pressure.
c. less heat needs to be supplied for the vapor pressure of the solid to equal that of the external pressure.
d. solids do not melt when heated under reduced pressure.
e. it prevents the pure product getting contaminated with impurities in the air.
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