Can water be heated until it emits visible light? I realise it won’t be liquid anymore - at least not at normal pressures - but can it be done without breaking the molecular bonds?
Breaking Water Down Into Constituent Elements:
Your steam will be glowing pretty good by the time it hits 2700 °F.
I don’t think water can be made incandescent in the sense of a hot black body emitter. When you burn hydrogen in oxygen, the flames are invisible but the flame product is gaseous H2O at 2000-3000 K depending on whether it’s burnt with oxygen or air.
I find it hard to believe that Szlater’s “luminous plasma ball” literally consists of “ionised water molecules”. In a plasma the hydrogen and oxygen will be dissociated from each other, and the electrons are free so they can undergo non-quantised transitions and emit visible light.
This pdf, page 4, explains that H2O doesn’t have the correct energy levels to emit visible or ultraviolet light. It’s infrared all the way for water.
http://www.tempe.mi.cnr.it/zizak/tutorial/cairol06-flame-emission.pdf
The UV given off in a hydrogen-oxygen flame comes from OH[sup]-[/sup] radicals.
That’ll teach me for not reading an article properly.
The same chap who claims that it’s ionised water molecules then says that it’s actually ionised hydrogen and oxygen, and some copper too.
D’oh!
So, how long would you steep your tea at these temps?
From the pdf:
Presumably an unappreciable strength is still larger than zero. I’ve looked, and can’t find values, but it’d be amazing if water at 2000°F didn’t have some visual emissivity.
vetbridge: Earl Grey, or Darjeeling? 
15.7 nanoseconds.
It’s actually the milk that is the problem.
One interesting thing that does happen is that water will eventually reach its critical point.
When I was a young lad studying boiler theory in a Navy school, I found it absolutely intriguing that, while the jump from 600psi to 1200psi in modern naval boilers was fine, you couldn’t just keep heating the water to get higher (and more efficient) pressures. Eventually you would hit 3200psi and your water would cease to be either liquid or gas.
It was never explained to me what would happen then. I imagined that the steam plant wouldn’t like stuff that wasn’t quite water nor steam flowing through it. And what would the stuff look like?
Mmmmm. A good Darjeeling, hold the moo.
Maybe you’re right. Hydrogen flames are apparently visible in the dark. Although since they are blue, there’s a good chance most or all of this is from the OH radicals in the flame rather than hot H2O.
H2O absorption is shown on this page (scroll down for a graph). I’m under the impression that if it can absorb a frequency then it can emit that frequency, so some visible light should be emitted. But I have no idea what those absorbance coefficients imply with regards to visible light.
Absorption and emition graphs often have some symmetry, but are not identical. I would not put much faith in this assumption. In this case the exitation would be thermal so only certain energy levels will become occupied. These energy levels may not be in the visual range.
Are there any other common substances that share this inability to emit visible light when heated? What is it about water?
Water molecules will remain intact at temperatures high enough that a non-negligible portion of the blackbody spectrum is in the visible range. So I would expect that there would be at least some excitation of those modes.
Am I wrong in thinking that the amount of radiation being emited as a blackbody would be reduced by the non-black body emisions in the infrared?
Of course, water will incandesce if you get it hot enough. It has a slightly bluish color and would have a useable emissivity in the visible red. It doesn’t dissociate until well hot enough to glow at visible wavelengths. The talk about spectral emissions isn’t relevant in blackbody thermal emission.
The critical point, above which temperature and pressure the liquid and gas states run together, is interesting. Right at the critical point, any value of density over a broad range is possible, and waves and ripples of density will meander about. Liquids slosh inside their containers, and gasses don’t - if you bring a container through the critical point of its contents, you will hear the sloshing get fainter and disappear. I have seen a beautiful little handheld demonstration device holding something - a refrigerant, maybe - whose critical point is between room temp and body temp. You see it act most amazingly as you warm it and wave it around.
Does everything and anything radiate as a black body when you get it hot enough? Why does a 2000 deg. C methane flame appear blue but a 3000 deg. C tungsten lightbulb filament appear yellow-white? If water has a useful emissivity in visible red, why do hydrogen flames appear blue?
Definitely feeling I’m missing something here!
Except that a body with spectral emisions isn’t a black body.