Chemistry · Ch 9 — Organic Chemistry – Some Basic Principles and Techniques
Sublimation
Sublimation
Sublimation as a Purification Technique
You already know that when you heat some solids, they turn directly into vapour without ever becoming a liquid. This direct solid-to-vapour transition is called sublimation, and the reverse process — vapour back to solid — is called deposition. The purification technique that exploits this behaviour is also called sublimation, and it is used to separate a sublimable compound from non-sublimable impurities.
The principle is straightforward: if your desired substance can sublime (like camphor, naphthalene, or iodine) but the impurities in it cannot, you can heat the mixture. The sublimable component will vaporise, leaving the non-sublimable impurities behind. The vapour is then cooled, and it deposits back as a pure solid on a cold surface.
Sublimation works only when the sublimable compound has a significant vapour pressure below its melting point. Many solids, even if they can sublime in principle, do so too slowly at safe temperatures to be useful for purification.
The textbook presents this as a single, clear idea: on heating, some solid substances change from the solid to the vapour state without passing through the liquid state. The purification technique based on this principle is known as sublimation and is used to separate sublimable compounds from non-sublimable impurities.
The Process in Practice
In the laboratory, the impure solid is placed in a china dish (or a beaker) and covered with a funnel that has its stem plugged with cotton. The funnel is inverted over the dish, and the assembly is heated gently on a sand bath or a water bath. The sublimable compound vaporises, rises, and deposits on the inner walls of the cool funnel as pure crystals. The non-sublimable impurities remain in the dish.
For better deposition, you can place a wet filter paper or a cold watch glass over the mouth of the funnel. The cold surface speeds up deposition and gives larger, purer crystals.
Common Examples of Sublimable Compounds
The textbook does not list these explicitly in this section, but from earlier chapters and practical knowledge, the compounds commonly purified by sublimation include:
- Camphor — the classic example, used in mothballs and traditional medicine.
- Naphthalene — also used in mothballs.
- Iodine — sublimes readily, giving violet vapours.
- Ammonium chloride — sublimes on heating, but note that it also decomposes partially; the vapour recombines on cooling.
- Benzoic acid — a common organic acid that sublimes gently.
- Salicylic acid — another organic acid that can be sublimed.
Not every solid that appears to sublime actually does so. Some compounds, like ammonium chloride, undergo thermal decomposition on heating and then recombine on cooling. This is not true sublimation, but the purification technique still works because the impurity does not volatilise. The textbook treats this as sublimation for practical purposes.
Key Idea Recap
The entire section boils down to one concept: if a compound can go directly from solid to vapour (sublime), you can separate it from non-sublimable impurities by heating. The vapour is then condensed back to pure solid on a cold surface. No formulas, no equations, no numbered properties — just a clear physical principle and its laboratory application. …