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Methods of Purification
Concepts tested here
- Purification of organic compounds
All Questions
2005 AIPMT 1 question
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The best method for the separation of naphthalene and benzoic acid from their mixture is -Sublimation is used to separate a substance that changes directly from the solid to the vapour state on heating from non-sublimable impurities.
Naphthalene sublimes readily on gentle warming (it is volatile even at room temperature, which is why moth balls slowly disappear).
According to the answer key of this paper, the mixture is separated by sublimation: on gentle heating the naphthalene sublimes first and is collected on a cool surface, leaving the benzoic acid behind.
Note: benzoic acid also sublimes at higher temperature, so in the laboratory the two are better separated chemically, by shaking the mixture with aqueous $NaHCO_3$; benzoic acid dissolves as sodium benzoate while naphthalene remains undissolved.
Distillation is suited to liquids, and chromatography is used mainly for small amounts or many components.
So the answer given by the key is sublimation.