June 05, 2020, 02:48:00 PM
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Topic: Why do the flames of my gas stove change their color when you move the pan?  (Read 14933 times)

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Offline gasherd

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Hi guys -- I am not a chemist and have wondered for a long time why the flames of my gas stove change their color from blue to yellow when you move the pan or pot over the flames.

Try the following:

-when cooking salt water, lift the lid, and let some of the water spill down near the flames. They will become yellow -- AND remain to be yellow for a much longer time than you would expect, i.e. as compared with just holding Na+Cl- into a flame (I am aware of that effect.

-what is much more surprising is, though, that if you place an empty pan on the stove and you knock the bottom of the pan or pot with some cutlery, the flames will also change their color from blue to yellow for a considerable time -- and here, no NaCl is involved. (Therefore, I wonder whether the reason for the first problem mentioned is possibly not in NaCl either.)

Thanks! Best, Martin (Princeton)

Offline 0000000

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Martin,

the colour of the flame is directly connected with it's temperature.
If one flame is blue and other is yellow that's mean that the blue one have higher temperature than the yellow one.
In your experiments you reduce the temperature of the flame that so it changes it's colour at the beginning form blue to yellow and after that the opposite
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Offline Borek

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the colour of the flame is directly connected with it's temperature.

Wrong! What about emission spectra? Sodium - yellow, potassium - violet, barium - green and so on?
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Offline gasherd

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Hi Tomce and Borek,

Thanks for your quick replies. The temperature answer makes intuitive sense, and of course there are the color spectra (NaCl = yellow).

I don't think your answers contradict each other, but I have not been compelled by either that it is the one and only true answer...

I have not heard of gas flames having different colors according to temperature. Is there some source you have for that?

Offline 0000000

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Borek just to remind you that we are talking about common fire ;) like the one in the kitchen or the BBQ one

Martin said that he is not a chemist that so I don explain that kinds of fire colour.

And my ansfer is mostly refer to this

-what is much more surprising is, though, that if you place an empty pan on the stove and you knock the bottom of the pan or pot with some cutlery, the flames will also change their color from blue to yellow for a considerable time -- and here, no NaCl is involved. (Therefore, I wonder whether the reason for the first problem mentioned is possibly not in NaCl either.)
My name is Bond, Covalent Bond

Offline Borek

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Borek just to remine you that we are talking about comon fire ;) like the one in the citchn or the BBQ one

It doesn't change situation - you are wrong :)

First of all - if we see flame that's because there is something emitting light in the flame. If the flame contains solid unburnt particles that don't emit anything on their own, flame color will depend on the temperature and it will more or less exactly follow emission of black body - the hotter it is the more bluish it'll be.

Now, amount of such light emitting particles in the gas stove flame is very low - some soot may appear, but if the burner is correctly working the soot becomes oxidized to carbon dioxide almost immediately. That's where you can see some 'black body' emission - the only source of which is hot solid. However, even traces of sodium - that are present almost everywhere - will emit enough light to mask it. In practice if you see yellow flame in your kitchen it is almost always because of sodium (unless you are chinese cook adding ingredients to a hot wok :) - flame is yellow then because of the heated soot).

gasherd: IMHO sodium is the only source of color in the flame. Note that sodium present on the metal surface will not emit light, but once it will get 'free' in the form of aerosol (can be solid!) and into flame - it will burst with light. I suppose that knocking the pan you remove some small amount of sodium compounds from the bottom.

Try to do the same with pan that was washed several times with DI (or RO) water.
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Offline billnotgatez

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I want to preface this by saying that Borek has corrected me several times in the past. I am a mere amateur scientist and by no means an authority. But, being the fool that I am, I will throw in my 2 cents.

When discussing fire / flame theory we can be right and at the same time be wrong. I have had some experience with combustion analysis and in the process found it complex and multifaceted. Therefore, you can be right about one thing and wrong because you did not include other things.

Many of us have seen the change in color in an open flame due to adding salt (read sodium) and then it goes from blue to yellow.

We also know that when a flame burns hotter it goes from yellow to blue to white (more or less). Hence, we get the term white-hot.

We also have observe that when we change the flow of air into the bottom of a Bunsen burner we go from yellow to blue as we increase the air flow.

We have seen logs have different color flames when burning that go from yellow to bluish back to yellow. Logs do have sodium in them (read salt) but there are many organic compounds as well.

If you have had experience with gas lanterns you know that the flame burns blue until the mantle is lowered over it and then it burns bright yellow. I do not think that sodium is involved for camping lanterns.

So I would think that the flame changes in color by putting a pot on top of a stove top flame is a combination of many factors, not just one.



Offline Borek

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If you have had experience with gas lanterns you know that the flame burns blue until the mantle is lowered over it and then it burns bright yellow. I do not think that sodium is involved for camping lanterns.

You are right - no sodium.

http://en.wikipedia.org/wiki/Gas_mantle

Red hot, white hot refer to light emitted by hot substances, their color can be approximated by black body radiation, see

http://en.wikipedia.org/wiki/Black_body

However, most real life substances differ from the black body (in terms of quality of the approximation), so their emmision spectra (and I mean thermal emission here) differ from the one described by Planck's law. It happens that substances used in the gas mantle differ in such a way that they emit more in the visible range at the price of lower emission in other ranges. That's OK, as in lamp we want as much of the energy as possible to be used for light, not for heating.

In the case of Bunsen burner flame color depends on the amount of air (oxygen) mixed with the gas, but most of th elight comes from the black-body radiation. If there is not enough oxygen, combustion is not complete, there is much more hot soot that can emit light. Soot is reasonably close to the real black body. Sure, if there is not enough oxygen the temperature is also lower so the flame is more yellow and red then white. Highest temperature that is possible in the Bunsen burner is in the range of 1500 C - that's still far from the really blue, which needs temperature in the range of several thousands. So the blue color visible is not from the black body radiation, I recall reading something about hydrogen emissions - but that's completely different story.

However - gas stove has burners that are made to use always the same mix of air/gas in the right proportions. You can't change these proportions easily - the easiest way I can think off is to boil over milk and not clean the stove later :) So as long as the burner is properly working, flame has rather constant temperature and it can change color only due to some other reasons. Sodium being one of these.

Then, don't treat me as an authority. I am known to be occasionally wrong :)
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Offline 0000000

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OK,
Borek's explanation is OK, that's something known by all the chemsits, true but may be hard to be explained to non-chemists in the way he has.
The theses with temperature depending colour is also true, depending on me :)
And every flame have, generally, 2 parts, one of the the part when the oxidation is taking place is yellow, and the part below them is blue (this you can see everyhere, in every flame) you can mace a pseudo-change of the colour by adding something which will increase the oxidation's part.
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