Sublimation is a change of state where a solid turns straight into a gas — it skips the liquid stage completely. Warm a crystal of iodine and you never see it melt into a puddle; it simply gives off a purple vapour.
The reverse change is called deposition: a gas turning straight back into a solid, again with no liquid in between. That is exactly what happens when the purple vapour touches the cold flask in this experiment.
Sublimation only works as a separation method when one solid in the mixture sublimes and the other is heat-stable (it stays solid when warmed). Here, the iodine escapes upwards as a vapour while the table salt does not change at all — so the two part company. If both solids sublimed, they would simply travel together and you would separate nothing.
A rising vapour will only turn back into a solid if it meets a surface cold enough to take its energy away. The round-bottom flask of cold water is a handy, curved, chilly "landing pad" sitting right in the beaker's mouth. The vapour deposits on its underside as fresh, pure crystals.
The solid that collects on the cool surface is called the sublimate. It is pure, because only the iodine ever left the mixture — the salt never made the journey. The crystals can simply be scraped off.
Turn the collar to close the air-hole first, then strike the sparkler at the top of the barrel. The gas lights with a yellow, wavy luminous flame — easy to see and safe, but cool and sooty. Opening the air-hole lets air mix with the gas: the flame turns blue — the non-luminous flame — which is hot enough to warm the mixture. Never light a Bunsen with the air-hole already open, or the flame can strike back down the barrel.
Iodine sublimes with only a little warming, so a small flame is enough. Strong heating wastes gas, can crack the glassware, and makes the vapour pour out faster than the cold flask can catch it.
| Change | From | To |
|---|---|---|
| Melting | solid | liquid |
| Boiling / evaporating | liquid | gas |
| Condensing | gas | liquid |
| Freezing | liquid | solid |
| Subliming | solid | gas (no liquid stage) |
| Depositing | gas | solid (no liquid stage) |