Answer 1:
I think I understand your questions, but
forgive me if I missed something. The formation
of H2O from H2 and
O2
(2H2 + O2 ->
2H2O + heat)
is a canonical example
of a combustion reaction. The reaction energy is
too high for a simple mixture of H2
and O2 to render H2O, so
we need a catalyst, a source of energy for the
reaction. A flame or a spark fits the bill. In a
rigid sphere filled with H2 and
O2, a spark would produce an
explosion (a fast release of heat and light) and
H2O vapor in the sphere, along with
the excess
of hydrogen or oxygen. The water vapor may
condense to liquid water, but only if the
temperature and pressure inside the sphere is
below the critical temperature and at high
enough of a pressure, respectively. The critical
temperature is the temperature above which the
liquid phase cannot exist, no matter what the
pressure. For water this temperature is at 705
degrees Fahrenheit.
Were you to break the sphere, you would still
find liquid water. Even though the pressure
would decrease, the temperature would decrease
as well. Depending on how dry the air was, it
would take a noticeable amount of time for the
liquid water to vaporize.
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