Newbie keg carbonation question

Hey there. Trying to wrap my head around something conceptually when it comes to kegging. I understand the idea of adding the CO2 to carbonate the beer. What I don't quite get is why isn't the beer at the end of the keg more carbonated than at the start. I.e. towards the end you have maybe 1 gallon of beer exposed to 4 gallons of co2 . Wouldn't this beer have much more co2 dissolved into it because there's a much greater column of co2 than when you have say 4 gallons of beer and 1 gallon of co2?

Feel free to get geeky with the answer...at one point I swear I was good at chemistry and physics but all that knowledge has escaped my brain over the years lol.

 

No. There's a limit to how much CO2 beer can dissolve at a given pressure.

 

To elaborate on what Newsman said. You have a pressure reducer. It keeps a stable pressure of co2 within the keg. A liquid can just absorb so and so much gas in a given temperature under a given pressure, in your case your beer, and co2, at the temperature you keep your keg.

Since the pressure is equal during your beer's whole lifespan in the keg, the beer itself will not absorb any more co2 than it can, even though the headspace contains more co2, but the pressure is the same. You'd need to increase pressure or lower the temperature to make the beer absorb more co2.

 

The presumptions are that you have constant pressure on the keg, unlimited supply of CO2, and constant temperature of the liquid, and have equilibrium of CO2 dissolved into liquid. The only way to change the dissovled CO2 in equilibrium liquid is to change temp, or pressure; for instance taking keg off gas to party and as it empties, the pressure reduces, possibly warms, making bottom of keg warmer liquid under less pressure have less dissolved CO2.

 

i can always sense when i'm withing four or five pints of the keg kicking as it gets much more carbonated all of a sudden. I figured given the same pressure on the co2, the lower volume of liquid in the keg dropped to a lower temp in my keezer since it is colder with just a little liquid in the bottom of the keg than when totally full. but it sure happens all of a sudden and has me cursing my setup until i get the dreaded burp of foam then the little light in my head goes on and i realize what was happening.

 

Ahhh, ok. I guess I wasn't considering how pressure would keep the gas dissolution rate the same. Fair enough, Thanks guys.