bp's science: why does a soda can explode when shaken? (v.1)

I have my brother to thank for this post. His understanding of physical science is above and beyond my own. He can conceptualize pretty much anything and then explain it in terms I can understand. Let's hope I can use his explanations so that you get it too.

Let's look at our soda can first. When you open up a can that has not been shaken, there's a small release of gas.

When you open up a can that's been shaken, their is a release of gas that creates a soda explosion (the one we all tried to avoid when trying to look cool at any social function we attended as teenagers).


Why does the first situation have a calm release and the other a fierce one when the amount of carbon dioxide (CO₂) in the can has not increased at all? The secret is bubbles.

Let's look at the following equation, (which for those of you who took any type of physical science, you'll remember memorizing):

PV = nRT

where P = pressure,
V = volume
n = moles of atoms/molecules of gas (in this case carbon dioxide gas)
R = a constant
T = temperature

Now let's take this equation and think of our soda can. When we open up a can of soda, we change the pressure inside of the can, in fact, we decrease the pressure. In both situations (not shaken vs. shaken) when we open the can we decrease the pressure the same amount. Some would say this goes against their original thought that when you shake a can, you are increasing the pressure inside (at least that is what I thought), but that is not the case. Let's look at the equation again, solving for pressure.

P = nRT/V

- When we shake a can, do we change the number of moles (n) of CO₂ in the can? No, in order to do that we would have to somehow pump carbon dioxide into the can.

- R does not change, because it is a constant.

- Some would argue that shaking up a can would change the temperature (T). But shaking a can for 5 seconds, even 10 seconds, the change in temperature is negligible.

- And finally, when we shake up a can, the volume (V) of that can does not change. It would have to decrease to increase the pressure, but it does not.

So if none of the above is changing, and the pressure is not increasing inside, what is causing the explosion?

The answer: when you shake a soda can it creates lots of little vortexes or vortices (think mini-tornadoes) inside the can. These mini-tornadoes have an area of lower pressure in the center (just like the calm in the eye of a storm during a hurricane). When you open a shaken can, miniscule bubbles of CO₂ in the liquid rush to the area of lower pressure, that is, the area inside the vortex. This is a characteristics of all gases, they prefer lower pressure areas. As the CO₂ rushes to the area of lower pressure, the miniscule bubbles expand greatly. Thus the explosion.

Now if you do a search on the internet as to why soda cans explode (like I did), you will most likely not find an answer that includes vortexes. Instead, you'll get a simple explanation of bubbles mixing in liquid and wanting to escape, which has some truth to it. However, if you do a search on why soda cans explode and include the word "vortex" in your search, you are bound to find this great article, which will lead you to the source paper for this research.