Mystery Substance Boils and Freezes at the Same Time

A student chemist in a lab was working with a liquid that exhibited unusual properties. The substance would switch between being a frozen solid to a boiling liquid.

What science could be at play here?

First, take a look at the video of the lab experiment to see the substance in action:

"During a mass spectrometry experiment in the laboratory, we observed this happening to an unknown fluid sample," the Reddit user going by Tenstone wrote, noting that the point of the experiment was to identify the sample.

"The flask is being evacuated, which drops the pressure and temperature. However, the idea is to vaporize the fluid by dropping the pressure so that we can [analyze] the vapour with the mass spectrometer. Interestingly, what I believe is happening is that the top of the liquid (which has surface tension so is less prone to vaporization) is freezing due to the temperature drop, while the liquid underneath is boiling due to the pressure drop. Is this correct?" the Redditor asked.

Image source: YouTube

A few subsequent commenters on Reddit speculated that the substance hit its "triple point," a point at which the three phases -- solid, liquid and gas -- occur at the same time.

"Chemist here. You were indeed freezing it at the same time you were vaporizing it," BantamBasher135 wrote. "As /u/m1ld pointed out, you found the triple point of this particular sample. The reason the bubbles were freeing first... well, there are a couple options. First I suspect there might have been some solvent left over. When that was evacuated the pure compound froze instantly. Second, the bubbling at the surface allows for easy re-orientation of the molecules, which allows them to form a crystal lattice without any additional energy expenditure. Additionally, the growth of the crystals on top of the liquid provided surface area--and especially sharp points-- for the liquid to then boil."

This video from Fluke Calibration details how a triple point is reached:

Though Tenstone might have better insight into what could be going on with the liquid, as of Sunday the Redditor wrote that he's still not sure what it is.

(H/T: io9)