[B_Rian]

Originally Posted by bybcous

It can't be from the moisture in the air. If there was enough moisture in the air to produce condensation then it would be condensing on everything. There would actually be less of it condensing on the tailpipe, because the tailpipe is quite warm.

In fact the water is generated by the combustion of the fuel in the car. It comes from the hydrogen in the fuel, plus some of the oxygen from the air. For example, the combustion of octane is

2C8H18+25O2→16CO2+18H2O+heat.

This is just the net result of an extremely complex series of reactions, and motor fuel is not just octane, but ultimately burning fuel in a car will produce carbon dioxide and water in roughly equal amounts, plus much smaller amounts of a whole bunch of other things.

Usually the*H2O*will be in the form of water vapour, but if it's cold then this will condense, and this is the liquid water you see coming out of the tailpipe.

that's what I was gonna say