I've noticed that when I first turn on the flame under a cool pot the sides immediately mist with condensation. Why?

The outside of the pot? If your flame is methane, CH4, than the reaction is

2[CH4] + 3[O2] --> 2[CO2] + 4[H2O]

That is, twice as much water as CO2. If your pot is cold enough, then this water could condense onto the sides.

I'm talking about the flame on my kitchen stove. I assume it's natural gas. Doesn't smell like farts when I turn it on without lighting it.

 

Try it yourself. You'll see what Im talking about.

Yes, natural gas is methane. Methane is odorless, although they do add something with a distinctive smell so that leaks can be noticed before your house blows up.

yep, methane by itself doesn't smell and never mind like farts.

 

farts smell due to sulphur dioxide and a couple of other compounds.

 

gas that gets piped to your housecontains trace(parts per trillion) amounts of methyl mercaptan (could be ethyl, not quite sure) and it is essentially the smelliest substance known to man. This has no effect on condensation forming on the outside of the pot.

 

you'll get condensation from any burning hydrocarbon, even if you put a big pot petrol under there and lit it.