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

Question

Just like this guy's, the color of my stove's flames were affected by the humidifier as well.

Why does this happen? Is it a good thing or a bad thing ?

Comments are not for extended discussion; this conversation has been moved to chat. Please note that any further comments which are not suggestions for improvement of the question, or requests to clarify it, are likely to be deleted. — David Z, Nov 19 '18 at 21:27
Given the explanation in the accepted answer, it'd be neat if you could post a third picture taken when the humidifier is filled with distilled water, which should lack the salts found in drinking water. Grocery stores often sell distilled water in gallon-sized plastic jugs. — Nat, Nov 20 '18 at 07:54
The pictures in the question are not by OP. He took them from the linked website. So no chance we'll get a third picture with distilled/deionized water. — user27542, Nov 20 '18 at 10:17
Can you confirm or clarify if the color of these flames appears to be the same as that caused by normal cooling of the flame? You could spray or drip water that is not from the the humidifier to see if the color is the same. Putting very cold water in a thin metal pot over a high flame should also cause water to condense on the pot and drip into the flames, causing an orange colored flame for a brief moment. It would be edifying to know if the orange colors are the same. Photos of both flames taken with the same camera might be interesting also (although not necessarily conclusive). — Todd Wilcox, Nov 20 '18 at 21:32
@ToddWilcox same camera doesn't guarantee anything at all. At the very least there must be fixed white balance to get consistent color reproduction, and low enough exposure to avoid blown-out red channel. — Ruslan, Nov 21 '18 at 12:01
Related. https://chemistry.stackexchange.com/questions/5000/butane-burning-color — , Nov 21 '18 at 18:20
I've linked to and borrowed from your question here.; I hope you don't mind. — uhoh, Mar 25 '20 at 06:29

Ruslan · Answer 1 · 2018-11-23T20:33:07.330

103

OK, this question appears to have generated some controversy. On the one hand is the answer by niels nielsen (currently accepted), which implies that the orange color is from sodium. On the other hand is the answer by StessenJ, which implies that the orange is normal black body radiation from the soot. Plus there are lots of commentators arguing about rightness or wrongness of the sodium answer.

The only good way to settle the matter is an experiment. I did it, with some modifications. First, instead of gas stove I used a jet lighter (ZL-3 ZENGAZ). Second, instead of humidifier I used a simple barber water spray. The third necessary component is a diffraction grating, a cheap one I had bought on AliExpress. I inserted it into colorless safety goggles to avoid necessity for a third hand.

When I lit the lighter I saw a set of images in the first diffraction order: violet, blue, green, yellow and some blurred dim red. So far consistent with the spectrum of blue flame given on Wikipedia. Then I sprayed water in the air, simultaneously moving the lighter trying to find the place where the flame will change color. As the flame got orange jets instead of initial blue, I noticed orange image of the flame appear between red and yellow images in the diffraction grating.

Below is a photo I could take with the grating attached to a photo camera's lens, having mounted the camera on a tripod and holding the lighter and spray in both hands while 10s exposure was in progress (sorry for bad quality). Notice the yellow/orange (colors are not calibrated) tall spike at the RHS: that is the part only present in the orange flame. (The jet indeed became visibly taller when it changed its color to orange.)

From this follows that the orange color indeed comes from sodium, otherwise the orange flame's image would be much wider and spread into multiple colors like the flame from a candle or a non-jet lighter.

The readers are welcome to replicate this experiment.

EDIT

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.)

What's worth noting is that not only the sodium 590 nm line is present in the orange flame, but also two potassium lines – 766 nm and 770 nm.

EDIT2

Just tried the same with a humidifier instead of the spray. The result with filtered tap water is the same: orange flame with sodium peak. With distilled water, although the experiment with the spray still resulted in orange flame (basically the same as with tap water), with the humidifier I got no orange at all.

Anyway, in no one case was I able to make the lighter emit continuous spectrum. Whenever I got orange flame, it always appeared to be sodium D doublet, not continuous spectrum.

edited Nov 23 '18 at 20:33

answered Nov 19 '18 at 20:27

Ruslan

28,862

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I suggest trying this experiment again, but using distilled instead of tap water. – David Hammen Nov 19 '18 at 20:35
Do it again with the tip of a knife. – Maury Markowitz Nov 19 '18 at 21:32
@MauryMarkowitz this was the first thing I tried (I did read your answer). I tried multiple metal things with the lighter, but they didn't even change flame's color. Dunno why, maybe gas pressure too high, maybe something else... – Ruslan Nov 19 '18 at 21:33
2

Very nice! Though by this experiment we can't really tell if it's sodium specifically. It might also be calcium. It would have been great to do the experiment with a more accurate spectrometer. – jkej Nov 20 '18 at 09:28
@jkej true, it would be really great if someone could do this. There are multiple challenges for this though: 1) having the necessary equipment to make both the flame and the water drops autonomously generated to free your hands (a stove and a humidifier would be OK for this), 2) having a very sensitive spectrometer or very intense flame to have useful SNR (notice that I had to use 10 s exposure at ISO 1600 with fully open aperture of f/5.6 to make the photo above), especially since decent spectral resolution is expected. – Ruslan Nov 20 '18 at 09:45
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@jkej as for your suggestion that this might be calcium, I don't think it's plausible. I only observed this orange spike blink with the flame, while calcium flame spectrum has lots of spectral lines, see e.g. here (taken from this page). – Ruslan Nov 20 '18 at 09:56
@Ruslan Yes, after inspecting the emission lines of sodium and calcium more closely, I agree that sodium makes more sense with the single line you're seeing. Still, the case would be even more convincing if we could establish the exact wavelength of the line(s). I have a very good spectrometer for this purpose, but unfortunately I don't have a burner or a spray bottle. – jkej Nov 20 '18 at 11:29
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@DavidHammen I've tried the experiment with distilled water (the bottle label reads "дистиллированная вода более 30%", whatever that "more than 30%" means...). The results are the same: I still do get the orange spike in the spectrum. Maybe the water is not pure enough, dunno... – Ruslan Nov 20 '18 at 18:29
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+1 for experimentation, but I don't think it's actually sodium. When I put a pot of cold water on a gas stove, the water vapor formed by the combustion of methane condenses on the bottom and sides of the cold water pot, until the pot becomes hot enough, of course. That's essentially distilled water, at least I hope we can agree it should have little to zero sodium dissolved or suspended in it. When that condensed water drips back into the flame or onto the burner, the flame briefly turns orange. That makes me pretty convinced it is cooling of the flame that causes the color change. – Todd Wilcox Nov 20 '18 at 21:26
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@ToddWilcox I've tried my lighter in the outdoor cold of 0°C, no orange flames were there, even when I moved it so that the flame bent. Also, 590 nm is a pretty good indicator of sodium, and now with the better-resolved measurements (see the spectra) I'm certain within ±2 nm that it's the sodium D line(s). As for your stove, it could have the remains of salts before the condensation, which would then dissolve in the condensate and get into the flame. – Ruslan Nov 20 '18 at 21:34
@Ruslan It's not the ambient air temperature that's the thing, it's the completeness of the combustion (or lack thereof). Another way to generate the non-sodium based orange flame is with a different fuel-air mixture. – Todd Wilcox Nov 20 '18 at 21:36
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@ToddWilcox I don't quite understand how you would get 590±2 nm line with incomplete combustion. What combustion product would emit this line? – Ruslan Nov 20 '18 at 21:54
@ToddWilcox The line at 590 nm is very obviously the sodium D line. And potassium is also there, could be from fingerprints. – Nov 20 '18 at 22:24
Relevant: https://www.nrl.navy.mil/content_images/05Chemical_Fleming.pdf The oxygen displacement argument looks pretty good to me. When the droplets evaporate, they expand by a factor of 1000. So the mass fraction of water in a nebulized mist can be significant, effecting both carburization stoichometry and combustion. Note the normal flame color of a ceramic propane heater and most other hydrocarbon diffusion flames. The premix system is definitely out of whack. – Phil Sweet Nov 21 '18 at 04:00
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@Ruslan Very nice indeed! That definitely settles it for me. Very interesting with the potassium lines too. It would be really nice if you could mark the theoretical wavelengths of the sodium and potassium lines with vertical lines in the spectrum plot, so that people could see how well they match. You might also want to point out that the lines at shorter wavelengths present in both spectra are the Swan bands. – jkej Nov 21 '18 at 09:28
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@ToddWilcox This experiment clearly refutes that it's black-body ration from soot. Your example with a macroscopic drop of condensed water falling into the flame is a very different situation. Nobody's disputing that cooling the flame can turn it yellow/orange, but that doesn't seem to be what's going on here. – jkej Nov 21 '18 at 09:40
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@PhilSweet But the effect was observed with the humidifier in a different room on a different floor. Surely, the water droplets would vaporize before reaching the kitchen. That is unless the whole house was saturated with water vapor. – jkej Nov 21 '18 at 09:44
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@jkej Where does that piece of info come from? – Phil Sweet Nov 21 '18 at 11:57
@PhilSweet from the blog post linked to in the question. – jkej Nov 21 '18 at 12:30
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My take on this experiment is that it isn’t using the same humidifier and conditions of the original question and it doesn’t exclude the possibility that an orange flame caused by something other than sodium in the water could have the same emission spectrum. Basically, it doesn’t reproduce the original conditions accurately enough to be conclusive, in my humble opinion. – Todd Wilcox Nov 21 '18 at 14:18
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@ToddWilcox It could certainly be improved, but this experiment clearly demonstrates that it is possible to induce sodium line emission in a gas flame to the extent that it appears orange by spraying the air with tap water. That goes a far way to settling the question in my book. Are you saying that something else than sodium might have caused the emission line that just happens to be at the right wavelength for sodium? Well, it certainly isn't black-body radiation from soot, and until someone comes up with a more plausible explanation, I think it's fair to assume that it's sodium. – jkej Nov 21 '18 at 14:53
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The fact that the distilled water produces the same result means that we only have part of the story. It might be that there was residual sodium in the apparatus; or that the filtering wasn't sufficient; or it might be that the sodium was introduced by the apparatus itself and not the water; and if left long enough (and with top ups of water) would stop producing the orange results.
"It's sodium" as a conclusion is very different to "it's sodium in the water"
– UKMonkey Nov 21 '18 at 17:50
@UKMonkey Fair point. But there seems to be some uncertainty whether it really was distilled water. Why would a bottle of distilled water say "more than 30%"? Further investigations with distilled water would certainly be interesting. – jkej Nov 21 '18 at 18:58
@Ruslan Are you sure the label on the bottle said "вода" and not "водка"? ;) – jkej Nov 21 '18 at 18:59
@jkej водка is 40% :D . Anyway, here's the label, if someone can read Russian: https://i.imgur.com/pzwIYA5.jpg – Ruslan Nov 21 '18 at 19:29
When I google "дистиллированная вода более 30%" I get ~5 hits for cleaning products and similar that list distilled water together with other ingredients. However, I get no hits if I replace 30 with another number. This leads me to believe that maybe there is a legal requirement to list it if it's more than 30%. Maybe that's why they write it like that even if it's 100% distilled water. What does the sentence directly after "дистиллированная вода более 30%." mean? If there were any other ingredients, I guess they would be listed there. – jkej Nov 21 '18 at 20:02
@jkej the next sentence isn't related to ingredients. It's instead "Doesn't irritate skin, mucous membranes of eyes and upper respiratory passages." Actually, after discussing here (page in Russian) I'm under the impression that this 30% is a copy-paste error by the designer of the label. The older version of the label doesn't have Ingredients ("Состав") section at all, so this 30% quirk can't be a legal requirement. – Ruslan Nov 21 '18 at 20:10
Well, either way it seems like this is most probably 100% distilled water. Maybe you could redo the experiment with this distilled water, but first clean the spray bottle with distilled water and pump a few times before you start the actual experiment to remove any tap water residue from the pump? – jkej Nov 21 '18 at 20:31
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@DavidHammen - OP's link: "Just bought a humidifier and used filtered water (from a water softener). Next day the flames were yellow/orange. Saw this web site and turned off the humidifier and waited for a few hours and the flames were back to blue. Went out and bought distilled water. Used the humidifier and no problems now. I’m spreading the word! THANKS" – Mazura Nov 22 '18 at 20:04
@Mazura Filtered water and distilled water are different things. Filtered water contains dissolved salts and minerals. Distilled water does not. – David Hammen Nov 22 '18 at 21:08
@jkej apparently, the contamination of distilled water was from the spray. With humidifier instead of the spray I got no orange when using distilled water. – Ruslan Nov 23 '18 at 20:34
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@Ruslan Ok, this definitely settles it. Great research and well deserved reward! – jkej Nov 25 '18 at 16:53
I've linked to and borrowed from your answer here.; I hope you don't mind. – uhoh Mar 25 '20 at 06:29

niels nielsen · Accepted Answer · 2018-11-19T10:02:04.787

76

The explanation I furnish below will stand or fall on the outcome of an experiment I and others here have suggested which is also outlined in my response. I promise to edit or delete my answer per the recommendations of the moderators here if that experiment shows it to be wrong.

Humidifiers that operate on the "cold" principle- mixing tiny droplets of water thrown from the blades of a fan with a blast of air- produce a mist of water vapor-enriched air mixed with the partially-dried remains of water droplets that are enriched in salts by evaporative attrition.

Those salt-enriched specks, when drawn into a hot gas flame, then emit light at frequencies corresponding to the line spectra of the salt constituents. In the case of sodium chloride (the most common salt in tap water), the sodium produces a yellow-orange glow when it hits the flame.

This phenomenon forms the basis of a chemical analysis technique called flame spectroscopy, in which a platinum wire is dipped into a solution containing an unknown mixture of salts, and then stuck into a hot flame. The colors emitted as the salts in the solution are heated are then used to identify the chemical constituents of those salts.

(Since sodium is ubiquitous, and this test is so sensitive to it, the platinum wire must be dipped in hydrochloric acid, heated to redness, quenched in the acid again and reheated several times to rid it of sodium before running the test on the sample.)

This mechanism can be ruled in our out by observing the flame through a grating that separates out the primary sodium line and I invite anyone here who has a gas range (which I do not) and a grating (which I also do not, sorry) to perform the experiment and report back to us here.

Since any dust in the kitchen would likely have salt in it, if the humidifier fan is blowing dust into the flame it would make the flame yellow as well. This can be tested by running the humidifier without water in it.

edited Nov 19 '18 at 10:02

answered Nov 18 '18 at 02:25

niels nielsen

92,630

10

How much salt is in your tap water? I find it unlikely that drinkable tap water would be brackish enough to show noticeable sodium colouring in a flame. – Anders Sandberg Nov 18 '18 at 12:18
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Hard water is mosly calcium and magnesium carbonates -- where is this supposed sodium chloride coming from? – David Richerby Nov 18 '18 at 12:25
@AndersSandberg It does not need many sodium atoms to give a flame a sodium color. But this is easily decided by taking a spectrum. Even the simplest will do: a prism, or a CD disk. And as Ilmari says, it could come from fingerprints. – Nov 18 '18 at 12:30
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@AndersSandberg: Then again, the sodium spectral line is very strong, and it takes very little salt to make it show up nice and bright. Back when I was doing flame tests in first-year inorganic chemistry lab, we'd spend quite a bit of time making sure to remove all sodium from the sample before even trying to test for any other elements, because if there was even the slightest trace of sodium left, it would overwhelm pretty much everything else. Oh, and don't touch the platinum wire with your fingers after cleaning it, because human sweat has lots of sodium in it. – Ilmari Karonen Nov 18 '18 at 12:30
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This answer is almost certainly wrong. You can get exactly the same effect by holding the tip of a knife in the flame, or any other object. – Maury Markowitz Nov 18 '18 at 13:51
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water which has been through an ion-exchange water softener has its calcium, iron and Mg ions exchanged for sodium ions. – niels nielsen Nov 18 '18 at 18:36
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to perform hot-wire flame spectroscopy in chem lab, we had to first heat the platinum wire to red heat and dip it in HCl repeatedly to get all the sodium off. even the tiniest amount on the wire would turn the bunsen burner flame yellow. Oops! I just revealed my age... – niels nielsen Nov 18 '18 at 18:50
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There is one observation that can explain all the argument in these comments. Niels Nielsen and Ilmari Karonen have commented on the strength of the sodium spectrum but it needs reiterated as it is the key to the argument here: the sodium spectrum is so strong that it masks others, even in water with a lot of Ca and MG and very little Na. The contamination of a knife you have touched is contaminated with your sweat. The Na simply overrides everything else unless you decontaminate thoroughly. But if anyone has a reference for how strong it is, please let as know as I can't find one. – David Robinson Nov 18 '18 at 22:30
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With all respect, the answer is almost certainty incorrect. The orange flame color is obtained in a gas flame in heating tubes in restaurants and public plazas by fuel-rich mixing. I took a pocket spectroscope to these, and their spectrum is indeed continuous. This is a thermal radiation from soot, not the Na line. I live near the ocean in a so salty air that all my tools corrode if I don't oil them for storage, as do guitar strings--and still my range flame is blue. I am very, very skeptical the OP's humidifier concentrates trace Na+ in air to express its telltale line suddenly so brightly! – kkm -still wary of SE promises Nov 18 '18 at 23:10
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@kkm : Then that simply gives two competing explanations, each of which would seem to produce the same result with a naive observing device like the eye and/or digital camera. The only way this could be analyzed more conclusively is, thus, to conduct an empirical investigation with an experiment to try and reproduce the effect as achieved with a humidifier nearby as in the OP,with spectrometer present to see if it is line emission or continuous (thermal) spectrum in that particular circumstance. It's entirely possible for the same (esp. naively) observed effect to have multiple etiologies. – The_Sympathizer Nov 19 '18 at 00:36
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@The_Sympathizer, amen to that! If only I were not a theorist... :) – kkm -still wary of SE promises Nov 19 '18 at 02:16
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@jkej, see the bold text portion of my last edit. – niels nielsen Nov 19 '18 at 10:04
After having googled this phenomenon and found that it's quite common and seems to be very specific to ultrasonic humidifiers, I have changed my mind and deleted my comments. This is very fascinating! It would still be very interesting to see this confirmed with spectroscopic measurements. – jkej Nov 19 '18 at 13:14
@DavidRicherby you're right, ultrasonic humidifiers are well known for depositing limescale on everything in the room, eg wrecking CD/DVD players. – Agent_L Nov 19 '18 at 16:24
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@Agent_L And according to Wikipedia, the emission from calcium is orange. That explanation seems more likely than sodium. – jkej Nov 19 '18 at 17:28
@kkm : The proposition is that the sodium present is not trace sodium in the air, but that the humidifier is releasing the sodium dissolved in the tap water. Measurement is needed of how long (in duration) the humidifier must be running before the flame changes to orange, and whether there's a critical distance from the flame beyond which the effect ceases, if we are to measure the humidifier's effectiveness at concentrating enough vapour to cause the effect. But if the flame is hot enough, it's hard to see why it would not react to dissolved salt still in suspension in the water vapour. – Ed999 Nov 28 '18 at 05:35
@kkm : You seem to imply that for the flame to go orange, it must be producing soot. The presence of soot in the flame implies that something is burning in the flame. If this is caused exclusively by running the humidifier, a further implication is that the emissions from it are being burned. However, the emissions appear to be restricted to water vapour and any impurities present in the tap water. Burning sodium chloride won't yield soot, which requires carbon. Did you intend to imply that the impurities must include sodium/calcium carbonate? And what is the evidence for sooty deposits? – Ed999 Nov 28 '18 at 05:51
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@Ed999, read Ruslan's answer. I was most likely wrong. – kkm -still wary of SE promises Nov 28 '18 at 06:54
My gas flame turned orange-red after I started using a cool-mist (ultrasonic) humidifier; turned blue when I stopped for a day; then returned to orange-red. I have a small direct-vision spectrometer and I was able to get a reading of the wavelength of 6000Å ±200 or so, so Na looks very likely. – Irl Concord Feb 10 '21 at 03:38

score 44 · Answer 3 · answered Nov 18 '18 at 06:51

44

The water cools the flame to the point where you get incomplete combustion, just like a candle. The yellow light is from glowing carbon, a.k.a. soot.

answered Nov 18 '18 at 06:51

StessenJ

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Soot gives a black-body spectrum, salt gives the Na atomic lines at 589 nm. So this would be easy to decide experimentally. Look at the flame through a grating. Or a CD disk. – Nov 18 '18 at 10:53
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Also, sodium yellow is a fairly recognizable flame colour. The more orange colour in the photo, assuming camera colour correction have not completely changed it, looks much more like incomplete combustion than sodium to me. – Anders Sandberg Nov 18 '18 at 12:17
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the original Golden Gate Bridge overhead lights were low-pressure sodium, which produced an intense orange color that is very similar to the orange tint in the OP's photo. modern high-pressure sodium lights are whiter in color because they have a thermal spectrum superimposed on the line spectrum. – niels nielsen Nov 18 '18 at 19:01
Natural gas is CH4 and its combustion goes to completion without producing significant amounts of carbon which would tint the flame yellow and orange, as in the case of a candle that is burning paraffin. – niels nielsen Nov 18 '18 at 19:13
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@AndersSandberg the camera often lies! (Though I agree with your conclusion) – Chris H Nov 19 '18 at 09:26
@nielsnielsen so what happens if you run a bunsen burner off natural gas with the collar closed? That's methane and give a yellow flame. Or the gas fire in my living room when turned up high, because yellow flames are decorative, even if inefficient (BTW it's a balanced flue, room sealed, so no CO worries). Answer on chemistry.se with more detail – Chris H Nov 19 '18 at 09:30
@Pieter: Sounds interesting. What would be the result of looking through a grating or a CD disk? – Eric Duminil Nov 19 '18 at 09:43
@chrish, that I did not do. The other thing I did not do was to expect 1) that this question would elicit this much interest, 2) that my answer would get upvoted as it did and 3) that it would trigger this much argument. My faith in my answer has dissolved by now and I regret having posted it. I'll try another edit of it. – niels nielsen Nov 19 '18 at 09:52
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@nielsnielsen I'm an experimentalist (in fact a spectroscopist) and a large part of me hopes that no one has done the experiment byt he time I get home tonight. I lack a spectrometer at home but dont lack old CDs, cameras, or lenses, so could bodge something. – Chris H Nov 19 '18 at 10:10
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@EricDuminil With a transmission grating in front of your eye (or in front of the lens of the mobile phone camera), you would see the flame spectra on both sides of the flame. If it is sodium, you would see exact images left and right of the flame with the same color. If it is soot, you would see bands from the red. Maybe I can try something here today with an ultrasonic humidifier and a Bunsen burner. But that would not be the same as your stove and your humidifier. – Nov 19 '18 at 13:21
A very quick test using a CD as a diffraction grating was inconclusive. The spectrum from salt on a wire appeared the same as that from splashing water on the hot cast iron centre of the gas burner (which should produce droplets, I don;t have a humdifier). I think it actually needs an entrance slit lens and camera, and in that case a non-curved grating would be good – Chris H Nov 19 '18 at 20:32
@ChrisH No, it does not need a slit, just some distance to the source, that would give enough angular resolution. Compare also with a candle flame (which is glowing soot). – Nov 19 '18 at 20:50
@Pieter, the experiment was limited by the length of my arms. Holding a steel wire in the flame for more than a few seconds let it get red-hot so I had to be able to reach to put it in the flame very briefly, while also hand-holding the rest. There wasn't enough light to project the image onto the wall. Ideally I'd have a pre-slit lens as well, to get more light into the system. I have a rather nice spectrometer on the bench behind my desk in work, but can't get a flame in that lab. – Chris H Nov 19 '18 at 20:57
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If the flame were caused by incandescent emission from incomplete combustion the question you have to answer is why it is so orange? A candle flame (designed to produce soot that will glow from heat and therefore emit lots of light) is far less orange despite being a much cooler flame that a cooker or burner. – matt_black Nov 19 '18 at 21:09
@StessenJ Actually, the results of measurement given in my answer disfavor this explanation. And one more data point: when I turn on my jet lighter outdoors (it's 0°C there), it still doesn't have any hints at orange flame, while this would be expected due to cold, if we follow your reasoning. – Ruslan Nov 20 '18 at 06:26
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The cooling effect from wet air (wet as in liquid droplets, not vapor) is greater than that of merely cold air. My theory is still that the water causes a cooler flame. – StessenJ Nov 20 '18 at 17:31
@StessenJ But the effect was observed with the humidifier in a different room on a different floor. Surely, the water droplets would vaporize before reaching the kitchen. That is unless the whole house was saturated with water vapor. – jkej Nov 20 '18 at 19:09
@nielsnielsen: Methane combustion in normal circumstances produces very little glowy soot, it is true. However, if the flame is cooled down to below-normal temperatures (by, for instance, spraying water into the flame or holding a cold spoon in it), soot does accumulate, and burn orange. – Vikki Nov 22 '18 at 01:28
@jkej: Isn't that the usual purpose of a humidifier? – Vikki Nov 22 '18 at 01:31
@Sean Is what the purpose of a humidifier? To saturate the whole house with water vapor? No, humidity levels close to 100% would have several negative effects. According to Wikipedia, a humidity level between 30% and 50% is recommended. If the humidity level was close to 100% they would most likely have been uncomfortable and turned off the humidifier. – jkej Nov 22 '18 at 10:23

score 14 · Answer 4 · answered Nov 18 '18 at 14:12

14

The accepted answer is not correct.

I have a gas stove in the basement which I have to do periodic maintenance on. This requires you to remove a bunch of fake logs, which are made out of some very lightweight material, I think something similar to rockwool but more solid. After maintenance I turn it on to be sure it's still working, and noticed that if the "logs" are not in, the flame is pure blue. This piqued my interest, so I replaced the logs and noticed that the flame turns orange after a short period, which corresponds visibly to the "logs" beginning to glow red. For instance, here is the stove shortly after starting (as quickly as I could run from the thermostat to the stove) and then again about two minutes later:

Not a huge difference, but you can see it. The flame along the front has no "log" over it so it remains blue. There used to be some rockwool insulation here but I removed it thinking it was left over from the installation.

It is not entirely clear how the "logs" do this, but it is clear this is purely due to the temperature of the flame. For further proof, I took these two photos of our cooktop:

As you can see, simply inserting something cold into the flame causes it to turn orange. Now a huge effect here either, but that's because I was one-handing the photo and the knife isn't properly positioned. The humidifier does this by inserting a mist of water over the entire area.

answered Nov 18 '18 at 14:12

Maury Markowitz

8,201

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Your theory is plausible, but your evidence is also explainable by sodium; the knife will have sodium on it (and is made of a bunch of other metals). How do you know it's the temperature and not the sodium (or other chemical) in both cases? Does the effect go away if you preheat the knife in the oven? – Richard Tingle Nov 18 '18 at 14:24
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"the knife will have sodium" - of come on, at the absolute best-case limit it will not be enough to be visible, and especially not enough to explain the fact that it will continue to be orange all day long if you care to hold it in that long. And what do you think is happening in the stove? Those "logs" are almost 30 years old and burn orange as long as the stove is running. The sodium explaination is completely implausible in both instances, or the hundreds of other objects you might try. Have you actually tried this, with anything? – Maury Markowitz Nov 18 '18 at 14:53
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Ilmari suggests the sodium line is very strong. https://physics.stackexchange.com/questions/441648/why-does-the-humidifier-make-a-stoves-flame-orange/441771?noredirect=1#comment991538_441651. And as long as the knife and logs continue to exist they are clearly made of something. Not saying you're wrong, just that you've not really proved your theory right either (and of course I haven't tried it, I'm neither the OP nor an answerer) – Richard Tingle Nov 18 '18 at 15:16
Not to mention the fact that the question isn't about a knife or logs but about a dehumifier which looks to have distinctly red flames – Richard Tingle Nov 18 '18 at 15:21
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@MauryMarkowitz "especially not enough to explain the fact that it will continue to be orange all day long if you care to hold it in that long" .. if the temperature of the object really was the reason, shouldn't it have heated up enough for the flame to lose that orange colour by then? – muru Nov 19 '18 at 07:04
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This is the most correct answer. Add a little more and I'll +1 you: Water droplets not only decreases the adiabatic flame temperature, as you say (perhaps to the point of causing improper reaction in and by itself). It also increases exhaust velocities for the same inlet velocities, at a lower temperature (steam production). As the velocities increase there will be a Reynolds number that threatens to cause turbulent flows, which would explain incomplete combustion - turbulence causes imbalance to a rather delicate fuel/oxidizer mix -> yellow flame. – Stian Nov 19 '18 at 09:32
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I have a suspicion that this answer is true -- a gentle breeze blowing on the flame will also cause its temperature to drop and the flame to turn orange (or at least it does on the butane stove in my campervan) – Chris H Nov 19 '18 at 10:13
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This might be correct; but the justification seems extremely weak to me. If the issue was with the temperature; why don't the logs start with an orange flame when the temperature distribution is greatest? Why does the knife keep the flame glowing red even when it's allowed to reach closer to equilibrium. I think this is closer than the accepted answer; but at the same time isn't really any more justified. – JMac Nov 19 '18 at 15:23
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@RichardTingle - the color of his flames is identical to mine, although that might not be obvious in the images where the colored part is very small. – Maury Markowitz Nov 19 '18 at 16:54
A second anecdotal piece of evidence from my parents' stove (I now have an electric one), that implies that this answer is correct: When the flames are small, they're pure blue. But increase the gas flow so the flames are larger and flare out a bit, and they turn orange without any other adjustment. Same shade as in OP's image, and OP's image also has them flaring out a bit from the humidifier's breeze. – Izkata Nov 19 '18 at 19:21
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@MauryMarkowitz You have clearly never tried to stop experiments being upset by sodium flame contamination. First, sodium is everywhere (glass, for example) and a very, very small amount gives a visibly detectable flame colour. The logs are probably made from a sodium-containing ceramic. Don't speculate about things you have no experience of. Every glassblower knows you are wrong in practice. – matt_black Nov 19 '18 at 21:15
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@matt_black - ok, suggest a suitable low-sodium material you would like me to insert into the flame and I'll be happy to take a photo. I will point out that simply fanning the flame also makes it turn color, and I'd be fascinated to learn how it is that slow-moving air does not contain sodium but fast moving air does. – Maury Markowitz Nov 19 '18 at 21:29
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@MauryMarkowitz Find some laboratory-clean platinum wire. And make sure the air in your house is totally free of dust (which contains a lot of sodium, mainly because you contain a lot of sodium).Fast air blows dust into the flame. Then you can trust the flame colour. And your experiment will be a well-controlled one. This is easy to verify in a lab with a spectrograph where sodium contamination from trace contaminants like dust is a constant problem. – matt_black Nov 19 '18 at 23:42
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The question reminds me of an explanation in a schoolbook. I'm also convinced that it's the temperature of the flame and not the sodium spectral lines that is relevant. At en.wikipedia.org/wiki/Bunsen_burner#Operation you can see how the flame color of a Bunsen burner depends on temperature. – Frank from Frankfurt Nov 20 '18 at 16:43
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This answer doesn't really say anything as to why humidifiers cause the flame to change color, it just shows that you can also partially change the color of a flame by inserting something into it, but does not prove that the mechanism by which the knife/logs change the color is the same as the one from the humidifier. – Herohtar Nov 20 '18 at 18:54
@Izkata The problem is that the heat flow causes winds to form, which carry the dust. Dust is everywhere, sodium is everywhere, and you only need absurdly tiny amounts of sodium to completely overwhelm the color of the flame, as anyone who ever did flame spectroscopy can tell you. If you want to prove anything, you need a test that distinguishes between the two - your test doesn't. Not to mention that of course, both can be true at the same time. – Luaan Nov 21 '18 at 09:01
Surely I don't have to quote first grade science?? Any kid knows that holding a poker in the fire will cause the metal to glow, and that it changes color as it changes temperature. With a butane gas jet, unlike a log fire or coal fire, the initial temperature is fixed: to ignite, the gas must be at a given temperature, its ignition temperature. And the burning mix of gas and air has a blue color reflecting that temperature. To boil off (or even evaporate) a mist of water droplets in the air will absorb heat energy from the flame, and the temperature reduction must show in the flame's color. – Ed999 Nov 28 '18 at 06:18

score 2 · Answer 5 · answered Nov 18 '18 at 14:06

Little bit of every thing here. But I would certainly not ignore CO. Remember gas comes out under pressure, and if any passes through the heat of the flame before being completely oxidized, you get CO. Presence of sodium could contribute, I would test with distilled water first. But water droplets would cool the flame faster by drawing heat to evaporate. Get a CO meter, and open the windows!

score 1 · Answer 6 · answered May 20 '23 at 07:03

Further supplemental/circumstantial evidence that the yellow part of a flame can come from sodium emission (as well as from the more commonly described thermal radiation from soot particles):

Chem Teacher Phil's YouTube video Making Black Fire with Table Salt shows/discusses a laboratory sodium lamp

...watch (them) warm up and make the entire room sepia tone because they're monochromatic which means they put out one kind of light you can also use them to make "black fire"

To do this I dissolved some salt and alcohol which when you burn it makes a nice orange yellow flame but watch what happens when I turn the (monochromatic sodium) light on the flame goes completely black, and this happens because both the light and the flame contain sodium ions which emit and absorb yellow light.

So as the bulb produces the light the flame absorbs it and looks black which is really weird to see in real life and looks almost fake.

Note: What's not mentioned explicitly here is that alcohol lamps not spiked with sodium generally burn with a blue flame - they don't make very much soot.

The mere possibility of this is already widely known from flame tests often demonstrated in school chemistry classes. — Ruslan, May 20 '23 at 07:14
@Ruslan your experience in school may not represent that of everyone - Stack Exchange has readers from all over the planet with all kinds of educational backgrounds. Perhaps your experience is on the privileged side of the planet's median. — uhoh, May 20 '23 at 07:50

score -2 · Answer 7 · answered Nov 18 '18 at 10:30

-2

The humidifier increases the percentage of water vapour in the air, which decreases the percentage of oxygen. The deficit of oxygen makes the burning of the gas less efficient, with the orange flame that also indicates a higher amount of CO in the exhaust gas.

Make sure to increase ventilation when you see orange tips on the flames.

answered Nov 18 '18 at 10:30

Douglas Held

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I doubt it, I find the explanation with the salt more likely. But this is easy to check experimentally by taking a spectrum: soot gives a black-body spectrum, salt gives the Na atomic lines at 589 nm. – Nov 18 '18 at 10:51
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The humidifier surely isn't displacing any significant amount of oxygen. – David Richerby Nov 18 '18 at 12:24
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If you're going to disagree with me, do it with metrics. – Douglas Held Nov 18 '18 at 16:45
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That's why there are so many reports of humidifier deaths, right? – user253751 Nov 19 '18 at 03:41
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@DouglasHeld You're the one who's claiming that dehumidifiers displace so much oxygen that a gas flame turns sooty. How about you provide some data to back that up? – David Richerby Nov 19 '18 at 16:29
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@DouglasHeld: There is 21 % of Oxygen in air. The saturation vapor pressure for humidity corresponds to approx 3 % of absolute concentration at room temp. (beyond that there will be "rain"). So there will not be a significant decrease of oxygen. It is not possible. Practically the humidifier will also not be able to come even close to the 3% (more likely between 1 - 2 %). – Andreas H. Nov 19 '18 at 16:32
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@immibis When a steam system, like in a ship, heating system or power plant, has a severe leak, people in the space die by asphyxiation. – user71659 Nov 19 '18 at 20:44
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@user71659 You can't compare pressurized pipework leaks with superheated vapour to a tabletop humidifying unit. Those are not humidifiers, and do not act like them. In that case, the pressure is built up far above atmospheric, so the vapour can take up a far higher percentage of the air which is kept at a lower pressure. You need situations like exploding heated and pressurized pipes in fairly enclosed spaces to have something like that happen. – JMac Nov 19 '18 at 21:05
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@JMac Both you and Andreas H make the flawed assumption that air coming from a humidifier is merely at 100% RH. That is incorrect. The air is supersaturated, just like the superheated steam, so the water condenses into an aerosol. This is why you can see the "steam" droplets from the humidifier. – user71659 Nov 19 '18 at 21:26
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@user71659 Are they not generally open to atmosphere and thus unable to really supersaturate? You would see droplets regardless because it would condense when it meets with the cooler air. You see vapour off a boiling pot which is open to atmosphere. I don't imagine many humidifiers would allow for significant superheating though. They are generally exposed to atmosphere AFAIK; that's basically how the vapour gets out. – JMac Nov 19 '18 at 22:24
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@JMac The chamber is significantly above room temperature. That's not vapor you see from a boiling pot, as water vapor is visually clear. It's an aerosol that has formed by condensation of the vapor. The aerosol particles are so small that air currents can keep them aloft, and they aren't dense enough that they coalesce into bigger droplets and rain. This is the principle behind clouds. You can also have mechanical action (ultrasonic humidifiers) that create an aerosol. – user71659 Nov 19 '18 at 22:41
@user71659 Okay, I get what you're saying now. I was thinking (and first only talked about) superheating, which doesn't really occur. The thing is, it should still condense the excess quite quickly. Any supersaturated steam would quickly condense as it mixes with the air to leave high humidity air. I don't think that would create much more oxygen loss, if really any at all. – JMac Nov 19 '18 at 23:03
So you are saying that in 100% relative humidity, the flame would be orange as well? That doesn't hold up with my experience :) – Luaan Nov 21 '18 at 09:05

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

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