Why can't we add charcoal to benzoic acid in purification by sublimation
I suppose you could but would it do any good? Charcoal is an agent to adsorb impurities in recrystallization. Sublimations doesn't use that technique.
Adding charcoal to benzoic acid during purification by sublimation is not recommended because it can interfere with the purification process. The reason behind this is that charcoal, which is a highly porous material, has the ability to adsorb impurities from a mixture, which is the basis of its use in various purification processes.
However, in the case of sublimation, the goal is to separate a pure solid compound from impurities by converting it directly from a solid to a gas and then back to a solid. Benzoic acid has a relatively low sublimation temperature, meaning it can undergo sublimation without decomposing. When heated, the benzoic acid vaporizes and condenses onto a cooler surface, resulting in purified benzoic acid crystals.
If charcoal were added to the mixture before sublimation, it would adsorb not only the impurities but also the benzoic acid itself, preventing it from sublimating and reducing the effectiveness of the purification process. Instead of achieving purified benzoic acid, you would end up trapping the compound within the charcoal.
To ensure successful purification by sublimation, it is best to avoid adding charcoal and instead focus on carefully controlling the temperature and condensation conditions to achieve pure benzoic acid crystals.