How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 1254

Why are there no bubbles in carbonated water until you open the sealed cap? Why are the bubbles inside the opened bottle then larger than the ones in a glass? — EP
When the bottle is sealed, its contents are in equilibrium. In this context, equilibrium means that while carbon dioxide gas molecules are continuously shifting from solution in the water to independence in the gas underneath the cap, there is no net movement of gas molecules between the two places. Since the company that bottled the water put a great many gas molecules in the bottle, the concentration of dissolved molecules in the water is high and so is the density of molecules in the gas under the cap. This high density of gaseous carbon dioxide molecules under the cap makes the pressure inside the bottle quite high, which is why the bottle's surface is taut and hard.

While you can't see it in this unopened bottle, there is activity both at the surface of the water and within the water. At the water's surface, carbon dioxide molecules are constantly leaving the water for the gas under the cap and returning from the gas under the cap to the water. The rates of departure and return are equal, so that nothing happens overall. Within the water, tiny bubbles are also forming occasionally. But these tiny bubbles, which nucleate through random fluctuations within the liquid or more often at defects in the bottle's walls, can't grow. Even though these bubbles contain gaseous carbon dioxide molecules, the molecules aren't dense enough to keep the bubbles from being crushed by the pressurized water. So these tiny bubbles form and collapse without ever becoming noticeable.

However, once you remove the top from the bottle, everything changes. The bottle's contents are no longer in equilibrium. To begin with, carbon dioxide molecules that leave the surface of the water are no longer replaced by molecules returning to the liquid. That's one reason why an opened bottle of carbonated water begins to lose its dissolved carbon dioxide and become "flat." Secondly, without its trapped portion of dense carbon dioxide gas, the bottle is no longer pressurized and it stops being taut and hard (assuming that it's made of plastic rather than gas). Thirdly, with the loss of pressure, the water in the bottle stops crushing the tiny gas bubbles that form within it. In fact, once one of those bubbles forms, carbon dioxide molecules can enter it from the liquid just as they enter the gas at the top of the bottle. As a result, each bubble that forms grows larger and larger. Since the gas in a bubble is less dense than water, the bubble begins to float upward until it reaches the top of the bottle. Because the bottle is taller than a typical water glass, a bubble has more time to grow before reaching the top in the bottle than it would have in the glass. That's one reason why the bubbles in a bottle are taller than in a glass. Another reason is that the concentration of dissolved carbon dioxide molecules is higher while the water is in the bottle than it is by the time the water reaches the glass, so that bubbles grow faster in the bottle than in the glass.

         

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