A flame is basically just gas that has been heated to a temperature where it becomes incandescent, right (e.g., a star)? So, is there a temperature at which water will produce such behavior?
It would have to be under pressure to stay liquid, but based on lava, molten metal, etc., I would presume that if it was hot enough water could be incandescent.
Yes, but it won’t be steam any more - it will be a collection of Hydrogen and Oxygen ions.
No. A flame is caused by combustion, a chemical reaction. Heat it enough and the gas will ionise, not only dissociating the hydrogen atoms from the oxygen atoms, but also the electrons from the nuclei. This is a plasma, which is what you see on stars
If you spray water into a gas grill with a spritzer bottle, the blue flames turn orange. Same thing?
Not really. You’ve just cooled the plasma with the spritzer.
You might find the Wikipedia article on Blackbody Radiation useful. I’ve set the link to go to the section on subjective colors, which shows you - for any object - what color it will appear due to heat. (Obviously, it could be a different color based on reflected light). This set of colors works for everything from ovens to lava to stars and is only dependent on temperature.
Thus, the answer to “Will X glow at heat Y?” is only dependent on the Y. The only reason X might be an issue is that X may or may not exist at the temperature in question. (It’s kind of like Cecil’s answer on melting wood - long before it melts, it stops being what we’d call wood.)
I know that steam in engines and reactors can be up over 1000 degrees. I haven’t ever heard of it being ionized in that case, but I suppose it’s possible.
I was going to say superheated steam is colorless, but I guess you colud go past that.
It’s worth noting that solids and liquids can be incandescent, and plasmas can emit electromagnetic radiation, but un-ionized gases do not. A hydrogen-oxygen flame can produce visible light from the chemical reaction of the hydrogen and oxygen, but the resulting water vapor can be at thousands of degrees Centigrade and still be completely invisible.
Most visible flames are from the incandescence of tiny cabon soot particles before they oxidize away. The gas mantle invented in the 19th century was based on the realization that you could make a fire hot enough that it produced very little flame, and then use that to heat a mantle to incandescence, producing much more light than you would otherwise get.
Doesn’t that assume that X behaves approximately as a black body? I would assume that water vapor doesn’t since it doesn’t absorb anywhere near all of the radiation that falls on its surface (at least at ordinary temperatures). But I’m no physicist.
You need to get water considerably hotter than ‘red hot’ to make it decompose into hydrogen and oxygen:
http://www.hionsolar.com/n-hion96.htm
So you should be able to heat water to about the same color temperature as an incandescent light bulb, 2700-3300°K, before it starts dissociating.
Now some materials are better at emitting blackbody-like radiation than others*, but I don’t think any, including water, are so unemissive as to not glow visibly when you heat them to 3300°K (5500°F).
*see Thermal emissivity