Preparation of Crystals by Sublimation
To prepare crystals of iodine, sulphur and ammonium chloride by sublimation.
1. Ammonium chloride
4. Test tubes
5. Bunsen burner
6. Watch glass
7. Beaker of water
8. Glass funnel
9. Hand lens
Heat portions of iodine, ammonium chloride, and sulphur in separate dry tubes and note what happens. Examine the products under a hand lens. Place some water in a beaker and place on it a watch glass containing solid iodine. Invert the glass funnel on the watch glass, heat the water to boiling and note what happens. Examine the product with a hand lens as before.
In the first part of the experiment, ammonium chloride turns to white vapour without passing through the liquid state, and condenses as colourless crystals at the cooler end of the test tube, again, without passing through the liquid state. Iodine does the same as ammonium chloride, but the vapour is violet and the crystals are shining purple. But sulphur first melts into a yellow liquid which turns darker and sticky until it boils to give a yellow vapour. This condenses on the cooler sides of the test tube into fine yellow crystals called "flowers of sulphur", but does not pass through the liquid state.
The three substances are therefore said to sublime. Sublimation therefore offers another method of preparing crystals. There is another thing for which the process of sublimation is important. If either ammonium chloride or iodine were mixed with a salt that does not sublime, e.g. common salt, then it would be easy to separate the two and thus purify them by sublimation. Sublimation therefore provides another method of purifying substances or separating mixtures of substances.