There is a device called a Fizz Keeper that attaches to carbonated beverage bottles and is supposed to keep them from going flat after they are opened by pressurizing them with air. Dalton's law and Henry's law have been used to debunk the device as ineffective. The argument against the device's effectiveness goes something like this: Henry's law states that the concentration of a gas dissolved in a liquid is proportional to the gas's partial pressure. Since the concentration of carbon dioxide in the atmosphere is low, the partial pressure of carbon dioxide contributed by the pressurized air is insignificant. Therefore, about the same amount of carbon dioxide must still come out of solution to reach an equilibrium partial pressure in a resealed bottle. But there's something I don't understand. According to Wikipedia, the carbon dioxide in the headspace of an unopened soft drink bottle exerts a pressure of about 2 atmospheres. When the bottle is opened, that carbon dioxide escapes and is replaced with air. If the bottle is closed again, carbon dioxide will come out of solution until its partial pressure again reaches 2 atmospheres (actually a bit less because the concentration of carbon dioxide in solution has decreased some, but let's call it 2 atmospheres for simplicity). But now we also have air in the headspace at 1 atmosphere, so why wouldn't the total pressure be 3 atmospheres (the sum of the partial pressures per Dalton's law) at equilibrium? The total pressure doesn't really go up 50% after a bottle opened and resealed, does it? Can anyone alleviate my confusion?