SolutionsSublimation
1. What sublimes?
Why can caffeine or iodine be purified by sublimation, whereas sodium chloride cannot?
Caffeine and iodine are molecular solids with an appreciable vapour pressure below their melting point. In sodium chloride, an ionic solid, the forces between the ions are so strong that its vapour pressure is negligible.
2. The cold finger
What is the “cold finger” of a sublimation apparatus for? What is the advantage of working under vacuum?
It is a cold surface on which the vapour turns directly into solid (deposition), so the pure compound collects on it. The vacuum lets the solid sublime at a lower temperature, without melting or decomposing.
3. In everyday life
Give two examples of substances that sublime at or near room temperature and pressure.
Dry ice (solid CO2), which sublimes at −78 °C, and naphthalene (mothballs) or the p-dichlorobenzene used in moth repellents.