How's this for a "state" of confusion?
This YouTube video (above) features a flask of cyclohexane that's been put under a vacuum. As the pressure in the flask is lowered, the cyclohexane on top begins to freeze as the cyclohexane below boils, simultaneously waffling between a solid and a liquid state.
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What's going on here? We asked a couple of experts and got a couple of answers.
The long answer:
What's happening, I think, is that, as the vacuum pump pumps, the pressure drops until it reaches a pressure corresponding to the boiling point of cyclohexane at room temperature. The liquid starts boiling but evaporation of the liquid requires energy (the so-called latent heat of vaporization). So the liquid near the surface cools and because cyclohexane has quite a high melting point, the liquid freezes. Then, because there is a layer of solid on the surface, the boiling stops. Then, either the heat from the warmer liquid lower down or because of cracking of the solid layer, the liquid starts boiling again. The whole situation may be further complicated if the experimenters didn't remove dissolved air from the liquid before they started because then the air will bubble out as the pressure drops. Eventually, the temperature and pressure may reach the so-called triple point, where the liquid, solid and vapour are in equilibrium. But it's not clear If they reached it here despite the title of the video.
Got that? Here's the short answer:
The liquid cyclohexane is "boiling due to [the] vacuum, and freezing due to evaporative cooling at the same time," Rick Sachleben, a research fellow in chemical development at Cambridge, Mass.-based Momenta Pharmaceuticals, told The Huffington Post in an email.