Question

How does sublimation purify a substance? What materials are removed?

Answer 1

Sublimation: This is a process of converting a solid directly to a gas, without going through the liquid phase, i.e melting.

So, you can use it to purify because while other compounds melt (or even remain solid) your compound will sublime to a gas which lets you seperate it.

By subliming Iodine, the gaseous Iodine will be separated from less volatile impurities, which may melt, but will not vaporise. By then depositing the gaseous Iodine on a cold surface, the impurities will have been left behind, yielding purer Iodine.