Why does carbonated water keeps it’s gas under pressure and loses it otherwise?
In: Chemistry
ELI5: carbonation is gas inside a liquid. Gas gets to the edge of the liquid. If there is no pressure to keep it in, it leaves.
The gas is carbon dioxide (CO2) and is both dissolved and chemically bound in the water. This is reversible: add more gas (pressure), then more gets into the water; remove gas and some will get out to replace the missing one.
This exchange is relatively slow, so it does not happen instantly. A bottle left open will slowly try to equalize the gas content with the atmosphere. But the atmosphere contains only very little CO2, so effectively all of it gets out given enough time. If you shake the water, this process is fastened a lot because the gas gets more chances to do so.
If you let a heavily shaken carbonated drink sit for some hours, the gas will get back into the liquid and it won’t all spill out when opened. Similarly those devices that carbonate water just push it in with high pressure, so it gets dissolved fast.
An interesting titbit is that this process depends on the _partial pressure_: the pressure exerted by the CO2 only, the other chemicals don’t matter. That’s why “fizz-keepers” are mostly pointless or slightly dangerous. They pump in normal air, which has only very little CO2, so the CO2 from the water still gets out until the CO2 itself is responsible for enough counter-pressure. But that adds to the already present high pressure from the fizz-keeper, potentially popping the entire bottle.
It’s always possible to dissolve a certain amount of other stuff into the water, but it’s pretty low for the carbon dioxide gas and it won’t feel “fizzy” that way. So the gas has been forced into water, much more than how much the water can take it normally. This isn’t natural, so the gas always wants to go out if it gets the chance. If you leave the bottle or the can open, that’s what happens. But if there’s outside pressure forcing it from doing that, it has no option than just staying in the water.
so co2 dissolving in water is a reversible reaction, the co2 can dissolve and can also come out of solution
normally in air the concentration of co2 is fairly low (in absolute terms) so very little dissolves in the water
if the co2 is concentrated and at higher pressure then more will dissolve into the water than what escapes, and this is how we carbonate stuff
if on the other hand you seal a bottle the co2 will start coming out of solution, until the pressure increases, and that will make more dissolve in the water until the rate of co2 dissolving and escaping match, in that case we reach equilibrium and the co2 level stays constant, even though it’s still constantly and slowly dissolving in and out of solution
Because CO2 doesn’t dissolve well in water at the pressure you and I normally live in. When we make carbonated water for cans or bottles, we shove the CO2 in at very high pressure so that it will be very fizzy. At normal temperatures, only a relatively tiny amount of CO2 actually dissolves into the water.
The fizziness of carbonated water happens specifically *because* the water can’t hold onto the CO2 mixed in with it.
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