The Friday Question: Fizzy Drinks

This Friday’s Friday Question comes from Gemma, and it is… “Why does shaking a carbonated drink make it fizzier?”

The answer is all about physics, which is pretty awesome. If slightly unnerving to a molecular structural bioloy nerd, it’s not often I fully understand physics!

The fizz in fizzy drinks comes from carbon dioxide, which is forced into the liquid under high pressure. The pressure means that there is a lot of carbon dioxide gas trapped into a small space; gas molecules whizz and bounce around in the neck of the bottle, and hit the surface of the liquid below, where they dissolve. Some of them escape back out again into the neck of the bottle. For a time, everything inside the bottle is manic and chaotic, molecules are bouncing around crazily, hitting each other, hitting the water, fleeing the water, generally acting a bit out of control. Over time, though, things settle down. An equilibrium establishes itself, this means that the rate of carbon dioxide leaving and entering the liquid is exactly the same; everything is stable.

If you opened the bottle at this point, you’d hear a hissing sound as the pressure was released, but that’s it, you wouldn’t get accidentally coated in fizzy drink. And then, once the drink was exposed to the Great Wide World, and all the air in it, all the carbon dioxide would eventually flee the liquid and escape into the air. Carbon just wants to be free, and this is why fizzy drinks go flat.

So what does shaking do to change everything? The bottle is sealed, so shaking makes no difference to the pressure. However, what shaking DOES do is disturb the liquid. Liquid has a surface tension; this means that the molecules in it have a tendency to be really friendly and stick together. It takes energy to prise the super friendly liquid molecules apart, which is what you need to do to form a bubble. Because liquid is SO friendly, overfriendly to be honest, the actual process of starting a bubble is Very Hard Work. Bubbles are frankly exhausted by the time they even come into being. Unless you give them a helping hand, in the form of a good old shake.

Shaking shifts everything around, and all a bubble needs to get started is the opportunity to find a crack in the surface tension of the liquid. A teeny tiny microscopic crack will do it, such as might be formed if liquid was bashed against an microscopically uneven surface, like the side of a bottle or can. Baby bubbles form in the liquid. Once a baby bubble exists, it takes a lot less energy for it to grow into a bigger bubble. It’s always the first step that’s the hardest to take.

Shaking also shifts the equilibirum; and some of the carbon dioxide gas that was in the neck of the bottle or the top oft he can, is mixed up with the liquid and can join any baby bubbles that have bravely managed to form. This means that that liquid suddenly contains a lot more bubbles, and if you open the bottle at THIS point, the change in pressure will cause  all these bubbles to rush to the surface and escape. Very quickly. Into your face, and all over your clothes. We’ve all been there, I think.


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