# What range of relative humidity is present in the combustion byproducts of a natural gas stove, immediately after combustion?

If I have a traditional gas range-top, burning natural gas (instead of LPG or propane), what is the typical range of humidity present in the combustion byproducts immediately above the flame? The combustion process adds water to the air, and the natural gas has an inherent humidity as well. However, the combustion process also dramatically raises the temperature, thus significantly drying the air.

• Why would there be any relative humidity immediately after combustion? Temperature is higher then boiling point of water. Mar 16 at 21:01
• @Mithoron No problem as long as there still exists a saturation pressure at that temperature. For example, pure steam at 120 °C and 1 bar has a relative humidity of about 50 %. Whether this is useful information, is a different question. Mar 16 at 21:11
• Whoa, generally in SE we don't downvote without leaving explanations, esp. for new contributors. If the question is dumb, please explain why. FWIW, it looks like there is an answer, which is that it is undefined. Mar 16 at 21:33
• Well, you just shouldn't ask about relative, but absolute or specific humidity. Relative one is highly dependent on temperature, even if it was below critical temperature. We don't really know what you want to find out here, I'm afraid. Mar 17 at 0:24
• @Mithoron thanks for the clarification. I'm curious if the combustion process makes the air generally wetter or generally drier. I've taken classes in thermodynamics related to powerplants, so I've never worked with temperatures so high (or low) that we've discussed critical points as something other than curiosities. Mar 17 at 0:33

Assuming ideal conditions starting from dry air, the theoretical mixture immediately after complete combustion contains $$19\ \%$$ $$\ce{H2O}$$ (molar fraction) since the oxygen from the air is replaced according to
$$\ce{CH4 + 2 O2 -> CO2 + 2 H2O}$$
However, the adiabatic flame temperature in air (not oxygen) is $$2236\ \mathrm K$$, which is well above the critical point temperature of $$647\ \mathrm K$$. Therefore, the relative humidity (i.e. the ratio of the partial pressure of water vapor to the saturation vapor pressure) is not defined at such temperatures immediately after combustion.