Bottled soft drinks that lose their fizz also lose their appeal. But it seems technology has come to the rescue with the “Fizz Keeper.” This little pump can be fitted into the neck of the bottle and with a few deft strokes the air space above the liquid can be pressurized. Soon the bottle feels as rigid as a fresh, unopened one. A pretty convincing effect. But does this device really put an end to the disappointment of finding a flat beverage in the fridge? I’ll let that question simmer while we take a little detour into the fascinating world of bubbles.
Bubbles don’t get enough recognition. They make bread rise, they’re responsible for the holes in Swiss cheese, for the texture of a marshmallow, for the bite of a soft drink, for the cracking of knuckles and for the fizzing of an Alka-Seltzer tablet. They can give the "bends" to divers surfacing too fast, they can make us burp (or worse), and they can cause geysers to erupt. But to most people, bubbles signal the boiling of a pot, whether it is watched or not.
Why does a pot boil? The water in closest contact with the heat conducting surfaces changes into a gas, and forms a bubble of water vapor, which being less dense than liquid water, rises towards the surface. When a bubble forms, it must of course displace some of the liquid, and this in turn must raise the level of the water. The level must be raised against the pressure of the air pushing down on the surface. Consequently, boiling can only commence when the pressure inside a bubble is equal to the atmospheric pressure. Since the pressure inside a bubble is proportional to the temperature, it is easy to see why water boils more easily at the top of a mountain than at sea level. There is less "atmosphere" sitting on top of it, less pressure to overcome. But since the boiling now takes place at a lower temperature, cooking times must be extended. Woe to the working husbands and wives of Denver, Colorado, elevation one mile!
Conversely, cooking times can be shortened by increasing the pressure on top of the water, as in a pressure cooker. A higher temperature is now needed to overcome the pressure created by not allowing the steam produced to escape. This then brings us to geysers, which are basically just natural pressure cookers occurring in areas of underground volcanic activity. As water percolates deep into the ground, it becomes extremely hot, but is unable to form bubbles. To form a bubble, as we have seen, water has to be displaced; in this case all of the water between the bubble and the surface has to be lifted to make room for the water vapor. This is just too much pressure to overcome, so the water becomes "superheated," and begins to rise up through the cooler water above, much the same as hot air rises over cooler air. Eventually, the hot water reaches a level where the pressure is such that boiling can begin. The superheated water bursts into an energetic boil and forces all the water above it to gush from the geyser. The expelled water will then slowly percolate down through the soil, and the old cycle can start over again… faithfully.
Bubbles can even form inside our bodies. Knuckle cracking, for example, is caused by exploding bubbles. The fluid which surrounds our joints contains dissolved gases. When a joint is stretched, the pressure on the fluid is reduced and the gases can "pop" out of solution. It takes about fifteen minutes for these gases to redissolve, explaining why there is a refractory period before the same knuckle can be cracked again. Cracking of knuckles is not dangerous, but the formation of bubbles in the blood can be a serious matter.
Scuba divers must breathe compressed air to overcome the pressure on the lungs exerted by the water on top of them. Under such pressures, the nitrogen which comprises eighty percent of the air they breath becomes more soluble in the blood than under ordinary conditions. If the diver now surfaces too rapidly, the nitrogen gas will bubble out of solution as the pressure is reduced. These bubbles can interfere with blood flow and cause the painful and potentially lethal condition known as "the bends." Coming to the surface slowly allows for a controlled liberation of the nitrogen and reduces risk. Still, some nitrogen remains dissolved in the blood. That’s why it’s not a good idea to dive and then fly in an airplane on the same day. The reduced pressure in the plane can lead to the release of residual nitrogen bubbles. The problem of bends, especially for deep dives, can be avoided by replacing the nitrogen in the tank with helium which is much less soluble in the blood.
Sometimes bubbles can also come in handy. Swiss cheese benefits from the production of carbon dioxide during the aging process. The holes form as carbon dioxide bubbles are released into the setting cheese. Carbon dioxide is also formed by the action of yeast on starch and thus causes bread to rise and champagne to bubble. The tingling sensation we enjoy when drinking a carbonated beverage is of course also due to carbon dioxide. Bubbles grow and expand into the small air spaces on the tongue and in the throat, thereby causing a pleasurable pressure sensation. Carbon dioxide bubbles can even have a theatrical effect. When a piece of solid carbon dioxide, or dry ice, is immersed in warm water, the heat vaporizes the substance, producing vigorous, impressive bubbling. That’s how stage hands can make “Double, double, toil and trouble, fire burn and cauldron bubble!”
Speaking about theatrics, one can generate carbon dioxide gas by reacting baking soda with an acid. This reaction is used in the classic school science project to make a “volcano” as well as in a variety of medications which fizz when dissolved in water. The “Plop, plop, fizz, fizz” effect is solely designed to please people who believe that a bubbling solution is somehow more effective. It isn’t. And I’m afraid that the Fizz Keeper also falls into the theatrical category. For you see, the manufacturers are obviously not familiar with Henry’s Law.
William Henry was an English chemist (1775-1836) who noted that the solubility of a gas in a liquid is proportional to the pressure of that gas over the solution. Other gases above the solution do not matter. So, consider a carbonated beverage. Before the bottle is sealed, it is pressurized with a mixture of air and carbon dioxide. The pressure of the carbon dioxide above the liquid is very high, far higher than atmospheric pressure, and a great deal of carbon dioxide dissolves. When the bottle is opened, the pressurized gas escapes. Now only atmospheric carbon dioxide remains to exert a pressure on the solution and this pressure is minuscule. Excess carbon dioxide therefore escapes, producing the fizz. Once the bottle has been opened, the only way that loss of dissolved carbon dioxide can be prevented is by repressurizing with carbon dioxide, not with air. Sorry for bursting the Fizz Keeper bubble.
