We've all had the experience of opening or pouring a carbonated drink, only to find that some numpty had been a little too rough in handling it and having the drink explode, spitting fluids all over the table, our clothes, and our friends.
Whenever you open that can or bottle, there is a small amount of pressure slipping into the surrounding, relatively low-pressure air. This happens because the matter in a high-pressure system, once opened and exposed to a low-pressure system, will inevitably take whatever path is available to it in order to equalize the pressure into a single system of uniform pressure. Nature abhors a vacuum, but it loves equilibrium. Simply put, the laws of physics trend toward balance. They don't like it when a lot of energy is concentrated in one spot and will keep things moving and changing until the system reaches a state of balance.
But why is a soda can under pressure in the first place? Why on earth would a simple mixture of water, sugar, and carbon dioxide react like nitroglycerine at the slightest provocation?
To start, let's figure out what soda actually is. Soda is what you would call a "solution". Not the solution to any problem mind you, unless maybe your problem is not having diabetes. A solution, chemically speaking, is any kind of mixture where one or more chemicals are dissolved in another. A solution consists of a solvent (a chemical that dissolves other chemicals) and a solute (a chemical that gets dissolved). In soda, the solution is a mixture of water (the solvent), and several solutes like sugar or another sweetener, any number of flavorings and dyes, and finally, carbon dioxide.
So what's happening on a molecular level when a solution of soda is created?
Every atom has a certain level of electric charge. It could be a positive charge, a negative charge, or neutral. Similar to magnets, opposite charges will attract each other, and like charges will repel. Since molecules are made of more than one atom, you'll sometimes have both charges in different parts of the molecule, creating a flow of energy that can attract other molecules
Water even does this to itself. The positively-charged hydrogen tails of the water molecules will attract to the negatively-charged oxygen atoms of other molecules. Put a bunch of them together, and they will create complex chains and webs of loosely connected molecules. This is is what causes surface tension, a tendency for water molecules to stick together. It's why falling onto water from a great height won't be much better than landing on solid ground. Your body goes from high speed into a sheet of water takes a little time to separate from itself, and since it doesn't separate as quickly as air, you slow down dramatically, impacting your body with lethal g-forces.
A web of interconnected water molecules.
When carbon dioxide dissolves in water at a low temperature and high pressure, this web also includes C02 molecules. Some of the negatively-charged hydrogen tails will attract the positively-charged oxygen parts of the C02 molecules. Now, water is bonded to water, creating surface tension, but also bonded to C02, keeping the carbon dioxide loosely trapped among a dog pile of water molecules.
CO2 dissolved in water.
When these C02 molecules are kept at the right temperature and pressure, it's relatively easier to keep them trapped within the solution. However, when the pressure drops and/or the temperature rises, the water molecules are moving around a little to fast to maintain the same level of surface tension, allowing C02 molecules to come out of solution.
Another thing about solutions is that if there is too much solute for the amount of solvent, it tends to get more difficult to add more solute to the mix because the molecules of the solute are more likely to group together and behave like they usually do when out of solution. Think of how bubbly soda is. Every bubble that fizzles to the top is another bit of C02 that has come out of solution. The soda is so oversaturated with C02 that dissolved molecules of C02 find each other, come out of solution, and rise to the top, escaping the soda.
You might notice that the bubbles seem smaller at the bottom of a glass of soda, getting progressively bigger as they rise to the top. This is because the bubble has created more surface area on the soda through which other C02 molecules can escape. This has a snowballing effect, as more and more C02 joins the bubble.
So why, then, does shaking a soda seem to speed up this process, and raise the pressure within a closed soda container?
Shaking a liquid creates turbulence. The small amount of air at the top of the container will start to mix with the soda, spreading little air bubbles throughout. By injecting these tiny air bubbles into the mixture, you create a ton of surface area on the fluid. Little pockets of air deep in the soda allow a convenient exit for the C02 to escape the solution, setting off a chain reaction as large bubbles create more surface area, allowing more C02 to come out of solution, etc.
Shaken soda with air pockets has more surface area, allowing more CO2 to escape solution.
Additionally, soda water is denser than an equivalent amount of water and C02 out of solution. A shaken soda jettisons the CO2 and creates two separate fluids, fluids that needed less space to coexist when they were part of a solution. If your can hasn't been opened yet, this is bad news for the C02, which has been evicted from it's home and has nowhere else to go. The higher volume of fluids pushes on all sides of its aluminum prison trying in vain to find somewhere to go.
Suddenly, an unknowing soda lover grabs the can and cracks the tab, breaking the seal of the can by creating a small exit point at the top. The high pressured system within the can joins the much lower pressured system of the atmosphere around you and seeks to equalize the pressure within the new single system. Translation: sugary water and C02 rush the door like angry soccer moms at a Black Friday sale, pushing and shoving past each other in droves like it's their only chance for discounted Furbys. Your shirt, along with any semblance of your pride, is ruined in a sticky mess.
(Left) Pressure building in closed, shaken can. (Right) Open can releases pressure and sprays soda.
If this post has made you thirsty, and you want to support this blog, stock up on soda for your fridge using the links below! Try not to shake them!