Obviously the hiss is co2 escaping from the bottle. But what I don’t get is why if I turn the cap just a little it’ll hiss then no more air will come out. Even if I squeeze the bottle no more air will come out. I can turn the cap a little more and get another hiss and I can repeat until all the gas is out.
In: Physics
Because, as you turn the cap you decrease the amount of pressure the cap can maintain. As gas escapes from the bottle, the pressure equalizes to what the (now partial) seal can maintain. Eventually the seal is completely broken and the pressure inside the bottle matches the pressure outside.
When you open it à bit the pressure from inside is pushing up on the cap pushing the seal away from the lip. Once a bit of pressure gets out the cap can come down a bit and reseal. Turning à bit more repeats this until the cap is far enough away from the lip to not form a seal
The seal which holds a bottlecap closed and airtight, gains a little bit of strength from adhesion between surfaces. The ‘stickiness’ of the soft rubberized cap insert against the lip of the bottle. This seal can be broken if there’s sufficient pressure from inside the bottle pushing out, but it’s a lot easier for escaping gas to *keep* an unsealed bottle unsealed, than it is for it to push out of a sealed one in the first place. Once the gap pinches shut, it tends to ‘stick’ shut, at least until disturbed. That’s why squeezing the bottle after it stops hissing, doesn’t necessarily squeeze any more gas out.
So when you twist the bottle cap, you’re doing two things. First, you’re loosening the cap overall, so the pressure required for the gas to push its way out is lower than it was. Second, you’re momentarily breaking the seal by moving the two plastic surfaces against each other. This means a bit of gas gets to escape for free, and then it just has to keep escaping.
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