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While reading about extraction of mercury from it's cinnabar ore (HgS), it's purification required distillation due remove impurities like Au, Ag, etc. This distillation was done at low pressure. Why is it done at low pressure?

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There is one obvious reason and one possible confusing factor.

The production of mercury from cinnabar involves heating cinnabar in a stream of air which causes oxidation of the sulphur and produces a stream of mercury vapour which can be condensed to give the metal. This isn't distillation even though it involves condensation of mercury vapour, but a chemical reaction.

But mercury can be purified by distillation because it is very volatile for a metal. But the boiling point at normal pressure is ~360 °C which is inconveniently hot. Using reduced pressure (say around 1/1000th of an atmosphere) can make this practical at closer to 100 °C which is far more convenient.

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