SublimationSubstances like ammonium chloride, naphthalene, camphor etc., change directly from solid to gaseous state on heating. For these substances sublimation technique is adopted. Sublimation is a process of conversion between the solid and the gaseous phases of matter.
Take a mixture of sand and iodine in a china dish. Cover the dish with an inverted funnel as shown in the below figure. Heat the mixture gently with a burner. Violet fumes of iodine vapour can be seen to rise through the funnel. The iodine vapour condenses to a solid on the cooler surface of the stem of the funnel. Stop the heating when no more vapour comes out of the mixture in the dish. The pure iodine crystals are scraped out. The non-sublimable sand remains in the dish.
Iodine is a common example of a substance that visibly sublimates at room temperature slowly. Carbon dioxide is another common example of a chemical compound that sublimates at atmospheric pressures - a block of solid CO2 (dry ice) set on a table will turn into gas without melting.