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I added urea to water and, predictably, observed a temperature drop (10 C). I wanted to place the beaker under (reasonably) high vacuum (for a classroom) and recover the urea as a dry substance. Well, I didn't. It's been a couple of months now, and there's white scale all the way up the inside, and crystaline filaments on the bottom, with an unchanging fraction of the original H2O. Is the substance still urea? There's been no odor. Is it ammonium carbamate? In either case, can I use vacuum to dessicate it?

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  • $\begingroup$ Let me see if I understood correctly - you put the aqueous solution of urea in a container (e.g. a vacuum desiccator), pulled a vacuum, then closed the container and left it to sit? If so, then it will take ages to dry. You need to continuously remove the moisture either by pumping the solution down constantly or by exposing the solution to a flow of dry air. Also, urea is very soluble in water, so removing the last bits of moisture will be difficult without heating. If you want to recover the urea to perform another endothermic dissolution, it may not work as well due to residual water. $\endgroup$ – Nicolau Saker Neto Jan 13 at 2:45
  • $\begingroup$ A vacuum desiccator does provide continual extraction. Nowhere does the author of the question state "closed the container and left it to sit". That may be the case, but it's an unfounded assumption. $\endgroup$ – DrMoishe Pippik Jan 13 at 18:24
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As water evaporates, the vapor pressure above the more concentrated urea solution drops. Either a better vacuum pump is needed, or perhaps gentle warming, to extract more water.

You might also try chilling to (partially) separate the solution. At ~260 K, achieved in many home refrigerators, much of the urea will have crystallized from the remaining saturated solution... just don't mix this up with food in the freezer.

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  • $\begingroup$ Thank you both. $\endgroup$ – Stumped Jan 14 at 18:24
  • $\begingroup$ You each stayed with "urea" and, by implication, stayed with physical change only, not a chemical one. That resolution was one of the answers i was looking for. Moishe, correct: open beaker. Thanks for the reminder about vapor pressure. $\endgroup$ – Stumped Jan 14 at 18:26

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