CDS (II) 2020 General Knowledge · Question 96 of 120

Correct — A, sublimation. The two solids differ in exactly one useful way: ammonium chloride passes straight from solid to vapour on gentle heating, while sodium chloride does not melt until about 801 degrees Celsius and stays put. Warm the mixture in a china dish under an inverted funnel and the ammonium chloride leaves as vapour and re-forms as a white solid on the cool funnel wall; the sodium chloride is left behind in the dish. Both components are recovered pure and neither is dissolved in anything. Every school laboratory sublimation demonstration is built on one of a short list of solids that behave this way — ammonium chloride, camphor, iodine, naphthalene and anthracene — and this item simply pairs one of them with a salt that does not.

• (b)filtration — Filtration separates an insoluble solid from a liquid. Here there is no liquid, and if water were added both salts would dissolve, so nothing would stay on the paper.
• (c)chromatography — Chromatography separates dissolved substances that travel at different rates over a stationary medium. It is a technique for analysing tiny quantities of coloured dyes, pigments and drugs, not for splitting bulk quantities of two white salts.
• (d)distillation — Distillation separates liquids by boiling and re-condensing them, and needs a difference in boiling point between two liquids. Neither substance here is a liquid at the outset, and ammonium chloride never becomes one — it goes straight to vapour.

Sublimation is the direct change from the solid state to the vapour state without an intervening liquid, and deposition is the reverse. It is used as a separation technique whenever one component of a solid mixture sublimes and the rest does not. Ammonium chloride is the usual demonstration solid: on heating it dissociates into ammonia and hydrogen chloride gas, which recombine into solid ammonium chloride on the cooler surface above, so the net effect looks exactly like sublimation and is taught as such.

Matching a mixture to a technique is always a matter of asking which physical property differs. Solubility differing points to filtration or crystallisation; boiling point differing points to distillation; density differing points to a separating funnel or decantation; magnetic behaviour differing points to a magnet. Here the property that differs is the tendency to sublime, and only one option exploits it. A useful check on the wrong answers: filtration and chromatography both need the mixture to be put into a liquid first, and putting these two salts into water dissolves both, destroying the separation rather than achieving it.

• Sublimation is the direct solid-to-vapour change; deposition is vapour to solid.
• The common sublimable solids in school chemistry are ammonium chloride, camphor, iodine, naphthalene, anthracene and solid carbon dioxide.
• Sodium chloride melts at about 801 degrees Celsius and does not sublime at ordinary laboratory temperatures.
• The standard apparatus is a china dish with an inverted funnel above it, plugged with cotton, so the vapour deposits on the cool inner surface of the funnel.
• Choice of separation technique follows the property that differs: solubility, boiling point, density, magnetism or the ability to sublime.

• Reaching for filtration whenever a mixture contains a solid, without first asking whether anything is insoluble.
• Confusing sublimation with evaporation — evaporation goes through the liquid state, sublimation skips it.
• Assuming distillation works on solids; it needs at least one component to be or become a liquid that boils.

As a mixture-to-technique match, or in reverse as 'which of the following sublimes', with camphor, naphthalene and iodine as the usual alternatives.

• practice — not a real PYQ

  Which one of the following substances sublimes on heating?

  • (a)Sodium chloride
  • (b)Camphor
  • (c)Copper sulphate
  • (d)Calcium carbonate

  Answer(b) Camphor — it passes directly from solid to vapour, which is why it disappears from a wardrobe without leaving a liquid behind.

• practice — not a real PYQ

  A mixture of sand and common salt is best separated by

  • (a)sublimation
  • (b)adding water, filtering and then evaporating the filtrate
  • (c)distillation
  • (d)using a magnet

  Answer(b) adding water, filtering and then evaporating the filtrate — salt dissolves and sand does not, so the difference exploited here is solubility, not the ability to sublime.