The cover photograph of the November 2014 issue of Chem 13 News (pictured above) is more interesting than it might appear at first glance. (Consider this demonstration 1.)
Normally one demonstrates magnesium burning by getting it started in the Bunsen flame, and then removing it so that it burns in air with a blinding white light. The product is a white smoke. This product can be collected by burning the magnesium under an inverted beaker, showing it is a solid, a white ash — MgO.
However, if the magnesium is not removed from the flame, as that photograph shows, then it still burns in the flame, not nearly so well (it sputters), and above it the flame is yellow-orange. The blue flame of the correctly adjusted burner, on its own, shows no such colour, nor does magnesium burning in air. Furthermore the product is not a pure white ash, but a white and black mixture.
This could be another Chemical Riddle — it is related to one
I wrote some years ago: “Chem Riddles, No.1”, Chem 13 News, September 2000, page 17.* Why does the magnesium burn at all in the flame? Why does it give out much less light? Where does the orange luminosity come from? What makes the ash black?
The "secret" to the riddle’s solution is the presence of CO2 as one of the combustion products of the hydrocarbon gas.
The reaction is as follows:
2Mg(s) + CO2(g) → 2MgO(s) + C(s) ΔH = -809 kJ
In comparison, the reaction in air is far more exothermic:
2Mg(s) + O2(g) → 2 MgO(s) ΔH = -1202 kJ
Hence the burning is less vigorous and gives out less light. The carbon formed gives off incandescent light in the flame (similar to the cause of the luminosity of a poorly adjusted Bunsen flame or a candle flame), and part of the carbon formed is left in the ash.
Incidentally, I would never have realized the potential of burning magnesium in the Bunsen flame, had it not been that I was showing potential teachers how to demonstrate the phenomenon in air, and one did it the "wrong" way (as above), and asked why the ash was black and white together. Like so many of the most interesting chemical experiences I have had over the years, it resulted from my allowing the students to do the experiment rather than me.
The following are a few other demonstrations in the same vein.
Observe a wire gauze lowered onto a cool flame. If you look from above, you can see that the flame is hollow.
As the gauze heats up, a flame reappears above it. Students eventually suggest that the centre of the flame contains unburned gas, and students may eventually suggest a test such as this:
Another interesting little experiment is to heat a thick piece of copper so that it blackens on removing it from the flame (CuO is formed); on putting it back in the flame, the shiny pinkish colour reappears on the surface as long as it is in the flame, but on withdrawing it, it turns black again. This effect results from unburned hydrocarbon gas (for example, butane) reacting with the oxide through the following reaction:
13CuO(s) + C4H10(g) → 4CO2(g) + 5H2O(g) + 13Cu(s)
If you thrust an unburned match into the center of the "cool" (luminous) flame, the part of the match in the center does not ignite, while the wood does burn! A burning match is extinguished when the head is put into the middle of the flame!
The students are always inquisitive to know what part the holes play in determining the flame color and heat. The drawing below illustrates a very simple way of showing that air is being drawn in (some students had thought that if the holes were opened, gas would come out). There is room for further discussion here (apart from the Bernouilli effect per se), regarding the changing color of the flame. Less air goes in, if the combustion products of the match take its place.
I published a riddle relating to the Bunsen burner flame in Chem13 News, February 2001, page 4.*
*Both of the riddles mentioned by Yehoshua will be posted online under “Supplemental materials” on the Chem13 News website.