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I don't know what exactly are the compounds that causes urine to be smelly, but does it (or do they) have higher or lower boiling points than water? If I have both liquid and exposed them to open air, will the water dry up first or will the smell disappear first?

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There are several reasons that a smell vanishes.

One obvious reason is that a smelly component has a very low boiling point and simply evaporates. Another reason might be a chemical reaction of the "stinker" when exposed to air, which converts it to something untraceable.

On the other hand, other reactions might result in quite the opposite but I rather doubt that you want to perform a test run with open bowls of urine over the next week at home ;)

Here is an older article from 1971 on the gaschromatographic analysis of volatile organic compounds in human urine after extraction with diethylether.

On a first view, however, the ketones that they found usually have a rather nice smell.

In order to get a hand on the components in the vapour phase above the urine, others have performed a headspace GC-MS analysis, see this application note and this article.

Again, the ketones usually have a pleasant smell, but they also identified some of the highly volatile bad boys, such as trimethylamine, methanthiol, and dimethylsulfide. These have boiling points of ~5, ~6 and ~40 °C, respectively.

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    $\begingroup$ It is important to note that unless the urine is sterile, its composition will change significantly over a few hours, as bacteria will grow feeding of the urea and producing ammonia. Therefore, even though other smelly volatiles are removed with time, ammonia is produced continuously and is tied with the exponential growth of bacteria, so older urine gets progressively more smelly. $\endgroup$ – Nicolau Saker Neto Jan 26 '14 at 14:16
  • $\begingroup$ @NicolauSakerNeto do you mean that left in the open, it will grow more smelly (because of the bacteria activity) instead of less smelly (because of evaporation)? $\endgroup$ – Olivier Beaumont Jan 26 '14 at 21:35
  • $\begingroup$ @OlivierBeaumont Precisely. Though I wouldn't really recommend you check it out yourself! $\endgroup$ – Nicolau Saker Neto Jan 26 '14 at 21:45
  • $\begingroup$ @NicolauSakerNeto oh god, why Now that explains what happened =)) I thought it was related to the boiling points. $\endgroup$ – Olivier Beaumont Jan 27 '14 at 2:53

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