let's say we pour some water to the Sun. Or boil water on 2000 degrees Celsius on Earth.
The water will probably turn into plasma really fast. I'm too lazy here to look up exactly how to figure this out but I can provide an outline to answering your question:
(1) Figure out how much energy is needed to split water into its atomic constituents.
a) Figure out how much energy is needed to pull two hydrogens off of oxygen. Then see if 2000 degrees C is more or less energy using the Temperature to Energy relationship, E = 0.5*kT, (E is average kinetic energy of particles in question, k is Boltzmann's constant, and T is temperature in Kelvins.) NOTE. This will give an approximate value of the energy in terms of temperature.
IF you have more than enough energy to do this, then use the remaining energy you have left and go onto part b.
b) Figure out how much energy is needed to ionize an electron from Hydrogen. Multiply that by 2, because you have two hydrogen atoms. Then figure out how much energy is needed to ionize all of Oxygen (Thats a lot of freaking energy). Then again, see how much energy you have left.
IF you still have more than enough energy, then move onto part c.
c) So by now you have 2 protons, 2 electrons from the 2 Hydrogens, and an Oxygen nucleus and its how ever many electron. You can then figure out how much energy is needed to break apart the Oxygen nucleus into free protons and neutrons, (THIS IS A LOT OF FREAKING ENERGY, and I don't know how to calculate this... good luck finding that.)
IF you still have more than enough energy, then move onto part d.
d) Figure out how much energy is needed to split protons into quarks, LOL. And see if you still have enough energy left. This is kinda overkill. Again, don't know how to do this. Sorry.
So if you figured out how much energy is needed to break one water molecule into whatever part you get to, b, c, or d. Multiply that by the number of water molecules you have to figure out the total amount of energy needed.
So in conclusion, its just simply comparing how much energy is needed to break apart water molecules and seeing what it turns into.
Well, take a look at liquid nitrogen - it just boils rapidly, and then it's just 'hot' gas, no matter how high is the temperature.
So if you pour water on sun - it will evaporate instantly, and you will have just steam at 6000 temperature (but in that range there will be molecular dissociation, but it's out of scope I guess).
Also, at high temperatures gas is likely to go into plasma state (i.e. became ionized).