Which one of the following methods can be used to separate anthracene from a mixture of salt and anthracene?
- (a)Distillation
- (b)Sublimation
- (c)Evaporation
- (d)Chromatography
Correct — B, Sublimation. Anthracene is a solid that sublimes, meaning it turns directly from solid to vapour on heating without melting, while common salt does not sublime. So gently heating the mixture drives the anthracene off as vapour, which then re-condenses as pure solid on a cooler surface, leaving the salt behind. Sublimation is the standard way to separate a subliming solid from a non-subliming one.
- (a)Distillation — Distillation separates liquids by boiling point; here we have a solid mixture with no liquid to boil off, so it does not apply.
- (c)Evaporation — Evaporation removes a solvent to recover a dissolved solid; it does not separate two solids like anthracene and salt from each other.
- (d)Chromatography — Chromatography separates components dissolved in a moving solvent by their differing adsorption; it is not the simple, direct method for a subliming solid mixed with salt, for which sublimation is designed.
Sublimation is the change of a solid straight into vapour without passing through the liquid state, reversing on cooling. It is used to separate a mixture when one component sublimes and the other does not. Anthracene, like camphor, naphthalene and ammonium chloride, sublimes, whereas common salt is left as residue.
The sorting rule is to match the property to the method. Only one component here sublimes, so the technique named after that property, sublimation, cleanly isolates it. Distillation and evaporation need a liquid phase, and chromatography is for finer solvent-based separations.
- Sublimation is the direct solid-to-vapour change, reversing to solid on cooling.
- Anthracene sublimes on heating, while common salt does not.
- Heating the mixture vaporises the anthracene, which re-condenses pure on a cool surface.
- Other subliming solids include camphor, naphthalene, iodine and ammonium chloride.
- Picking distillation or evaporation, which need a liquid, for a mixture of two solids.
- Overlooking that only one component sublimes, which is the whole basis of the separation.
Asked as which method separates a named subliming solid from a non-subliming one; the answer keys off the word sublimation.
No directly related past PYQ was found.
- practice — not a real PYQ
Which one of the following can be separated from sand by sublimation?
- (a)Common salt
- (b)Ammonium chloride
- (c)Sugar
- (d)Iron filings
Answer(b) Ammonium chloride — it sublimes on heating, while sand does not.
- practice — not a real PYQ
The direct change of a solid into vapour without becoming a liquid is called
- (a)Evaporation
- (b)Condensation
- (c)Sublimation
- (d)Distillation
Answer(c) Sublimation — solid changes straight to vapour and back to solid on cooling.