This is due to a reduction in atmospheric pressure – pressure due to the weight of the overlying air column). Remember that the higher the altitude the lower the air density and subsequently the lower the air pressure.
Therefore when heating water at higher altitude the vapor pressure of the water at the surface overpowers the atmospheric pressure – which is slightly lower than at sea level – even before the water temperatures reach 100 degrees centigrade.
Carbon comes from the air and soil, hydrogen comes from absorbing water, H20, and breaking it down into Hydrogen and oxygen :) (also where oxygen comes from)
