I hope this is the question you are asking about.. Feel free to ask again if it isn't.
I used to play with little plastic rocket toys when I was small.
They worked by putting some water in them and then pumping air as hard
as you could into them with a pump that was sold along with them. You
got it all pressured up, pointed it up in the sky (outside please! and
don't point it at anyone!) slid a plastic catch, and the water would
spray everywhere and the plastic rocket would go some distance into the
air. This is most fun on a hot, summer day. So when we say "bottle", I
am thinking of my old toy water rockets.
To optimize how far it would rise into the air, it is good to
experiment with different amounts of water in the bottle. The pressure
of the air and the water in the bottle will be the same (except for a
small hydrostatic difference caused by the weight of the water). What
matters is how much air and how much water there is. Water does not
compress very well, so if you fill the bottle entirely up with water,
it will not store very much energy when you pressurize it. The water
comes out of the bottle because the air expands when the end is
released, and if there is no air, then the water won't spray anywhere
and it won't be fun.
On the other hand, if you don't put any water in it at all, you
can squeeze lots of air in there which will all come rushing out when
you release the end, but air is not very dense, and it will not push
the bottle as hard as the same volume of water coming out.
What you need is some mixture of the two. I would suggest to experiment!!! It's fun!
(republished on 07/30/06)