Head space question and carbonation

Hey Everybody, I recently bottled a sparkling hard cider. When I cam to my final bottle I only had enough to fill about 2/3 of the 1litre bottle. Now I am using this bottle to check for carbonation before pasteurizing and I had a question. I opened the bottle after a few days to check on the carbonation level and I got a great pressure pop of CO2 from the bottle but there was ZERO carbonation in the cider. Is it possible that all the CO2 moved into the head space and that is why there was none in the cider?

In a word, yes. The CO2 doesn't like to go into solution, so when you gave it the extra headspace, that's where it went.

To expand just a little on Marauderhex's answer, the CO2 always goes into the headspace first. Only when that's been filled beyond a certain pressure will the CO2 start to go into solution and carbonate your beverage. The less headspace the faster it will carbonate, and too much headspace and it will never get there...

Thanks Guys, I am going to open a full bottle tonight to see if it is time to pasteurize.