General Question

shared3's avatar

Why do carbonated drinks go flat if their containers are opened?

Asked by shared3 (921points) March 26th, 2008

Is it because the pressure changes?

Observing members: 0 Composing members: 0

6 Answers

dpena2009's avatar

It’s because when the container is sealed carbon dioxide cannot escape. But as soon as the container is opened the bubbles of carbon dioxide rise to the top, pop, and release into the air, losing carbonation and thereby making the drink flat.

gooch's avatar

Yes they are under pressure therfore the gas is disolved into the solution. Once pressure reduction occurs the gases come out of solution and the temperature of the liquid drops.

richardhenry's avatar

The pressure inside a carbonated drinks bottle is much higher than the pressure outside the bottle. This helps keep the soda carbonated. That is, the additional pressure at the surface of the liquid inside the bottle forces the bubbles to stay dissolved within the soda.

When the bottle is opened, there is suddenly a great pressure differential. The initial loud hiss that is heard is this pressure differential equalizing itself. All of the additional pressure found within the bottle pushes gas out of the bottle until the pressure inside the bottle is the same as the pressure outside the bottle. The movement of this gas causes that initial loud hiss.

However, once this occurs, the pressure inside the bottle is much lower and the gas bubbles that had previously been dissolved into the soda have nothing holding them in the liquid anymore so they start rising out of the liquid. As they reach the surface, they pop and force small explosions of soda. These explosions are the source of the popping and hissing that continues while the soda is opened to the outside air. Of course, after a while, the soda will become “flat” when the only gas left dissolved in the liquid will be the gas that is held back by the relatively weak atmospheric pressure (when compared to the original pressure that existed inside the bottle while it was being shipped).

Interestingly, if you take a look at soda cans from the 60’s, you will see they were often much thicker than cans today in order to withstand the pressure inside. As technology improved, the cans could be manufactured out of thinner, cheaper and lighter aluminium.

winblowzxp's avatar

It really has nothing to do with pressure. A soda pop goes flat once you open it because it’s carbonic acid mixed in with the syrup. Carbonic acid (H2CO3) is quite unstable at room temperature, which is usually about 70°, and quickly breaks down into CO2+H2O which makes your soda go flat.

dpena2009's avatar

Hm… are you saying that pop has to be open to be room temperature? Because it doesn’t. Or that pop can’t get flat when it’s cold? Because it can. And the carbonic acid is already mixed in with the syrup, what gets out is carbon dioxide. Basically, what I’m saying is that what you said made absolutely no sense at all.

Jayy's avatar

This is related to chemical equilibrium involving le Chatelier’s principle. You’re quite correct in saying that pressure changes. When the bottle is opened the pressure decreases as the gas has essential the whole atmosphere to escape into. Now in a system in equilibrium, when this system undergoes a disturbance, the system will shift in such a way as to minimize the disturbance. This is known as le Chatelier’s principle. In the case of the carbonated drink, when the pressure DECREASES, from opening the bottle, equilibrium will shift to INCREASE the pressure according to le Chatelier’s principle. What this means is that carbon dioxide gas dissolved into the carbonated drink, will be released in an attempt to increase the pressure of the system. This results from gas being lost from within the carbonated drink thus the drink becoming flat.
A quick way to try and keep your drink being carbonated is to squeeze the container when you have drunk a bit and screw the cap on whilst the container is squeezed. What this does is reduce the volume of the container hence not as much dissolved carbon dioxide needs to escape from the drink in order to increase the pressure preserving the carbonation of your drink.

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