Someone is thinking smartly! Yes, indeed they can bend water vapor, as proved in "Return to Omashu". Aang blows on the chain and freezes the steam before shatterring it. However, Waterbenders don't often use steam, because it is wispy and insubstansial. I doubt they could make it hot enough to burn someone easily, because it involves to fine a control. They can, however cool it.

Oh wait, in Return to Omashu, he bends liquid water dissolved in air. Sorry no proof, though I do agree.

I was thinking aboutt hat the other day. But then I thought: well basically, aside from it's heat, steam is just tiny particles of water. So... Since waterbenders bend water in large quantities (for lack of better explaining on my part) As soon as they treid to bend the steam wouldn't it just gather all together and become just like normal water again? though it might retain the heat... I dunno. XD

I do agree that waterbenders could bend water vapor, as seen in the fortuneteller . Katara, with the help of Aang was able to bend clouds, which is mainly compressed water vapor. So water vapor is probably doable, at certain pressures and temperatures. But not steam.

The steam would have to be at high pressures and focused to noticeable damage to an opponent. The problem with steam is that if it is not focused or compressed it will dissipate its energy to the air, causing it to go through condensation. Molecules, especially water molecules, do not like heat or excess energy, which is why water has such short lived vapor state and such a long liquid state. So to keep it in steam at sea level, it would require massive amounts of controlled heat and pressure... simply unattainable by a waterbender.

Now... if you get a firebender, waterbender, and an airbender in a steel room, with the firebender controlling the temperature, and the airbender controlling the pressure, it would be possible but not plausible for the waterbender to bend steam. But then again… if you have all three in a steel room… there would be bigger problems then bending vapor ;D ;D

Yes, Waterbenders can bend water vapor. In the episode "The Drill" Sokka damages one of the pipes and causes water vapor to spew into the chamber. When a Fire Nation engineer came to fix the problem, Katara waterbended the water vapor around the engineer, condensed it to liquid, and then froze it into ice rendering him helpless while Sokka stole his schematics.

As Yi Feng pointed out though, you can't call that bending steam as the water vapor was visible and steam is not (I won't repeat what he has already listed as far as the science goes, but suffice it to say that what most people call steam technically is not). It is also hard to say how effectively water vapor can be utilized, or if simply condensing it as Katara did is really the only way waterbenders can manipulate it.

Copper for your thoughts.

Considering that Airbenders are the ones that deal with controlling gases perhaps it can be argued that if a water bender adopted so of the styles used in air bending then perhaps water benders could figure out how to effectively manipulate steam/water vapor.

Just like how Iroh adopted water bending ideals into his lighting redirect technique, I wonder if the arts of any one nation can be applied to all the different bends. This would be going along with how the Guru said that the nations are all one.

I know that waterbenders turn water into ice alot, but i don't remember seeing them ever really moving the ice after its formed. Maybe if a water bender adopted earth bending styles, [since earth deals with soilds] then perhaps a water bender could throw around ice the same way that earthbenders move rocks.

Sandbending already appears like earthbending which follows the air bending styles to manipulate the sand particles like air molecules.