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I burned them in a small aluminium tray. While IPA is burning orange, it produced the smell of soot, but while ethanol is burning blue there isn't any smell.

Also, ethanol made the tray really cold when I poured it on the tray before burning it, compared to isopropyl alcohol. How can this be explained?

Note: IPA concentration is 99%, Ethanol is 97%

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    $\begingroup$ Are you sure that your isopropanol is pure ? It does not look like so. $\endgroup$ – Maurice Mar 23 '20 at 12:44
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    $\begingroup$ What percentage isopropyl alcohol? 99.5%? 70%? 91%? What grade (industrial, laboratory, etc.)? $\endgroup$ – Peter Mortensen Mar 24 '20 at 5:25
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    $\begingroup$ The nice thing about very simple experiments, like this one, is that they can be repeated by others, if only to verify the reported observations. Unless this is done, explanations run the risk of getting too far in front of the skis. $\endgroup$ – Ed V Mar 24 '20 at 13:32
  • $\begingroup$ @PeterMortensen it would certainly have to be ruled out before proposing something else more complicated! I've added a supplementary answer borrowing from Physics SE. $\endgroup$ – uhoh Mar 25 '20 at 6:19
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IPA has a different carbon:hydrogen ratio than ethanol. There is more incomplete combustion occurring with IPA, hence the smoky orange flame and smell of soot. Ethanol combusts more completely, leading to a blue (soot-free) flame and no smell.

In response to your second question, ethanol likely has a lower latent heat of vaporisation than IPA, resulting in it evaporating rapidly. It takes a lot of heat energy away from the tray when doing this, leading to the tray cooling. A similar effect can be observed if you accidentally get some types of solvent on your hand and feel a sudden coldness as they vaporise, taking heat from your skin.

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    $\begingroup$ It has more to do with the length of the carbon chain and strong emission of C-C like radicals. Methylalkohol Burns by almost invisible flame. Erhylakohol with blue flame, sometimes with yellow in some flame parts. . IPA is expected to make more of yellow/orange color. $\endgroup$ – Poutnik Mar 23 '20 at 15:49
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    $\begingroup$ Yellow: There is always the possibility of "contamination" with sodium - it doesn't take much to make a flame yellow. $\endgroup$ – Peter Mortensen Mar 24 '20 at 4:55
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    $\begingroup$ @Peter Mortensen It is true. But I suppose one can distinguish it from the BB-like radiation by colour and mainly the spatial distribution of colour across the flame. And there is the mentioned "smell of soot", leading to C-C chains, what cannot be managed by sodium. $\endgroup$ – Poutnik Mar 24 '20 at 7:13
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Interesting observation. The blue flame color of all hydrocarbon fuels is due to the emission small diatomic carbon species such $C_2$ or CH. There is nothing magical about IPA having a yellow flame. The yellow flame originates from incomplete combustion. There is more carbon per mole of IPA as compared to ethanol. Yellow flames are called reducing flames and blue flames are called oxidizing flames.

In older times when Bunsen burner was taught in detail, it was shown a blue flame of methane can be readily converted in to a yellow flame by altering the air supply valve. The yellow color, if you view through a spectroscope is a continuous spectrum (rainbow like), which shows that it is like a black body radiator. The black body radiator is nothing but glowing soot (carbon) particle, glowing chrcoal but a very small one. On the other hand, the blue flame shows band like structure. I once had a chance to view the blue acetylene flame with air with a diffraction grating. It was an amazing sight. The structure of colored bands was never seen before. They are called Swan bands. Unfortunately, I cannot find any color images in Google Images of Swan bands.

Here is one example from a 1857 paper by Plucker and Hittrof, "I. On the spectra of ignited gases and vapours, with especial regard to the different spectra of the same elementary gaseous substance". This more than 150 year old picture is not doing justice to what you see in reality of an extremely beautiful spectrum. Swan bands

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    $\begingroup$ Swan bands are a topic in Flame Spectroscopy, Parts 1 and 2, by Radu Mavrodineanu and Henri Boiteux, Wiley, 1965. I do not have this classic book and it is extremely hard to get an original, though the reference above might be a later edition or reprint. Mavro used many exotic flame mixtures, including cyanogen and oxygen. I bet there are color plates in the book: they did a professional job in the old days! Maybe someone has the book and can check. $\endgroup$ – Ed V Mar 24 '20 at 1:23
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    $\begingroup$ Thank you Prof. Ed, Mavrodineanu's edited book on Analytical Flame Spectroscopy is online on Internet Archive. Sadly no color plates there. I checked Swan's 1857 original paper, no figures. However, I was shocked to see the comment in the book "Draper in 1848 [26], on looking through a spectroscope at the cyanogen flame, said: "There was a spectrum so beautiful, that it is impossible to describe it by words or depict it in colors." I cannot agree more. $\endgroup$ – M. Farooq Mar 24 '20 at 3:07
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    $\begingroup$ Yellow: There is always the possibility of "contamination" with sodium - it doesn't take much to make a flame yellow. I am surprised that an alcohol with only one more carbon atom compared to ethanol and nearly the same boiling point (78 °C vs. 83 °C) would exhibit this behaviour (stearic acid has an 18-carbon chain). The azeotrope with water is 88 wt% (96 wt% for ethanol). Isopropyl alcohol vapor is denser than air - could that play a role? $\endgroup$ – Peter Mortensen Mar 24 '20 at 5:18
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    $\begingroup$ Getting from the azeotrope to 99.5% may require some "chemical" means, potentially introducing small amounts of sodium. It seems one of the methods for breaking the azeotrope actually is adding NaCl (and destillation)... $\endgroup$ – Peter Mortensen Mar 24 '20 at 5:44
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    $\begingroup$ If we are assuming IPA was contaminated for some reasons, why should we assume the OP had ultrapure ethanol. He can never have access to absolute ethanol which is dry and pure. The student clearly mentioned soot which implied that it was a reducing flame. Oil flames produce a lot of soot too, I am not sure if you have seen old oil lamps. They are still used in some some Indian temples during worshiping. $\endgroup$ – M. Farooq Mar 24 '20 at 5:51
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Expanding on @PeterMortensen's comments (1, 2) here is some other discussion of how a small contamination of sodium can lead to orange flames:

From Why does the humidifier make a stove's flame orange?:

blue flame without humidifier yellow flame with humidifier nearby

From this answer to it:

enter image description here

OK, I've managed to measure some spectra using my Amadeus spectrometer with custom driver. I used 15 s integration time with the flame about 3-5 cm from the SMA905 connector on the spectrometer body.

Below the two spectra are superimposed, with the blue curve corresponding to the blue flame, and the orange one corresponds to the flame with some orange. I've filtered the data with 5-point moving average before plotting. The spectrometer has lower sensitivity near UV and IR, so disregard the noise there.

(Click the image for a larger version.)

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