I hope this question is appropriate for the Physics SE, although it involves a practical situation rather than inquiring purely about theory.

I've been trying to create carbonated water at home using a contraption involving two bottles connected by a tube; one is filled with the water to be carbonated (with the tube going almost all the way down), and the other has a mixture of citric acid, sodium bicarbonate and some water to dissolve the two (with the tube inserted just a bit lower than the cap).

The idea is simple, and has apparently been employed successfully by many people following an online tutorial (which otherwise used white vinegar instead of citric acid): produce CO2 that will go from one bottle to the other, while you're consistently shaking the water bottle to help dissolve the CO2. The tutorial said to do the actual carbonation until no more bubbles come out (probably around 30 seconds), give it another final shake and then let it sit for another minute, and finally open the carbonated water bottle while pinching the tube (supposedly it also helps keeping the CO2 in the water, but I think in practice it's only so you won't accidently have the acidic mixture from the other bottle spray into the drinking water).

I tried it several times using about 1.5tsp of sodium bicarbonate + ~equal amount of citric acid, and the water isn't left with any discernable fizz. I didn't find any smaller bottle, so the acid/sodbic bottle is 0.5 liter while the water's is 1 liter. The bottles already get quite stiff due to pressure (and I'm not sure how much more simple plastic bottles can withstand), but perhaps the pressure produced in practice using appliances such as Soda Stream is much higher, and nothing is dissolved in my experiment because there's simply not enough pressure? On the first attempt it shot the water all over the place when opening the bottle; on the last attempt, while opening carefully, nothing seemed to visually escape from the water but they were fizzless.

I used ice-cold water as advised.

Any suggestions? I did some calculations and the amount of CO2 produced from ~1tsp citric acid+1tsp sodium bicarbonate should at least nominally be enough to make 1 liter of water appropriately fizzing, if all of it were dissolved.

  • $\begingroup$ There is an equilibrium between the CO2 in the vapor space of your bottle and the liquid carbonic acid in the water of the bottle. To shift that equilibrium towards carbonic acid, you need to increase the pressure. Use cold water, and enough baking soda and vinegar to produce 2-3 atmospheres of pressure in the water bottle. Stay at that pressure for an extended period (e.g., 30 minutes). $\endgroup$ Jun 2 '19 at 21:16
  • $\begingroup$ From what I read, regular plastic water bottles should should withstand up to 150 PSI, so I suppose 2-3 atmospheric pressures should be fine. But I'm not sure of the process you're referring to in regards to maintaining that pressure for 30 minutes. Perhaps I don't understand what the said equilibrium between the vapor CO2 and dissolved CO2 actually refers to. When more CO2 is dissolved while the pressure is 2-3 atmospheres, will that pressure subside and thus I'll need to increase it again somehow? $\endgroup$
    – TLSO
    Jun 2 '19 at 22:50
  • $\begingroup$ In the past, I've put a good sized chunk of dry ice in a plastic bottle with 6-8 oz. of water in it and tightly sealed it. The plastic held the pressure, and when I removed the cap, the water fizzed. This obviously disturbed the equilibrium in the bottle, and if I wanted to keep the "fizz", I would need to keep the cap on the bottle, just as you do when you buy common carbonated beverages. If you can tolerate salt in your water, you can just dump the contents into the bottle and seal it. If not, you need to connect the bottles with a sealed connection. $\endgroup$ Jun 3 '19 at 1:59
  • $\begingroup$ I'm not following... If you wanted to keep the fizz, you would news to keep the cap on the bottle. But you need to drink it eventually — you meant that after keeping the cap on the bottle for an extended amount of time after producing all of the CO2 gas, more CO2 dissolved on its own and kept itself inside the water even after eventually opening the cap? And as I said, my current setup involves the two bottles (bottle with acid+sodium bicarbonate and bottle with water) being connected with a simple silicon tube passed through the caps. $\endgroup$
    – TLSO
    Jun 3 '19 at 8:20
  • $\begingroup$ The bottle pressures up when you seal it with dry ice inside. Some CO2 dissolves in the water at that pressure. When you remove the cap and reduce the pressure, some of that dissolved CO2 bubbles out of solution. Note that if you want to dissolve CO2 in water at atmospheric pressure, a small amount will indeed dissolve in water, but not enough to see the "fizz" that you are looking for. If you want a lot of "fizz", use the dry ice method, because that will give you the kind of pressure that you need. $\endgroup$ Jun 3 '19 at 17:41

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