Question

Explain how a process of fractional sublimation might be applied to a mixture of two substances which sublime closely together.

Answer 1

Explanation:

Sublimation is the transition of a substance directly from the solid to the gas state,[1] without passing through the liquid state.[2] Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point in its phase diagram, which corresponds to the lowest pressure at which the substance can exist as a liquid. The reverse process of sublimation is deposition or desublimation, in which a substance passes directly from a gas to a solid phase.[3] Sublimation has also been used as a generic term to describe a solid-to-gas transition (sublimation) followed by a gas-to-solid transition (deposition).[4] While a transition from liquid to gas is described as evaporation if it occurs below the boiling point of the liquid, and as boiling if it occurs at the boiling point, there is no such distinction within the solid-to-gas transition, which is always described as sublimation.

Answer 2

A process called fractional sublimation is used to separate mixtures of two or more materials with various sublimation temperatures. It is predicated on the idea that various compounds sublimate at various speeds and have various vapour pressures.

• Heating the combination to a level just below the more volatile substance's sublimation temperature is required for fractional sublimation. As the substance sublimates, the vapour is gathered and condenses back into a solid when it comes in contact with a cold surface. The more times this process is performed the purer a percentage of the more volatile material becomes after each cycle.
• It is necessary to carefully manage the temperature such that only one material sublimates at a time in order to use fractional sublimation to a mixture of two chemicals that sublime closely together. This can be accomplished by progressively raising the temperature while keeping an eye on how quickly each component sublimates.
• Consider a mixture of camphor and naphthalene, which both have identical sublimation temperatures. We can start by bringing the mixture to a temperature that is just below the temperature at which camphor the more volatile component sublimates.
• As camphor sublimes, its vapour is gathered and condenses back into a solid when it comes in contact with a cold surface. The residual solid mixture is then heated to a temperature that is marginally higher than naphthalene's sublimation temperature. This procedure is repeated multiple times with each cycle producing a fraction of camphor that is a little bit purer.
• From the beginning mixture, pure samples of both camphor and naphthalene can be extracted by carefully regulating the temperature and pressure conditions and repeating the procedure several times. Unfortunately, the method is not always practicable for large-scale separations because it can be time-consuming and necessitate specialised equipment.

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