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​Appliance Science: The compressed chemistry of carbonation

What puts the fizz into fizzy drinks? Appliance Science looks at the science behind carbonated drinks like soda, pop, Coke or Pepsi.

We pop the top of a can of soda or make our own sodas without considering what's going on inside. But sometimes, you need to stop and think, because these things that seem mundane are more complicated than you might think. There's a lot more chemistry going on in fizzy drinks than you might have thought. Let's take a look at the science of sparkling water.

Carbonation: It's a gas

For fizzy drinks like soda, the active ingredient is carbon dioxide (CO2). This colorless, tasteless gas is naturally present in the atmosphere in small amounts (about 0.04 percent) and plays a critical part in regulating temperatures. It's one of the greenhouse gases that absorbs infrared radiation from the sun, which helps control the amount of heat that reaches the surface of the Earth. Humans, animals and most bacteria breathe it out, and plants absorb it and use it to build sugars in photosynthesis, in a constant churn known as the carbon cycle.

The idea of carbonation isn't new. Beer has been around nearly as long as humans have, and this process produces CO2 that gives beer its foamy head. However, this process wasn't applied to non-brewed drinks until the 18th century. English chemist Joseph Priestley, the discoverer of oxygen, connected a bottle of water to a keg of brewing beer and noticed that some of the gas produced by the process dissolved into the water and was released when he opened the bottle. CO2 hadn't been identified at the time, so he called it fixed air. In the pamphlet Priestley released to announce his discovery (PDF), he suggested that water with fixed air didn't go sour like other water and that it might have medicinal uses. He later described it as his "happiest invention."

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The bubbles go up my nose

So, how does carbonation work? The basic process is forcing CO2 to dissolve in water. This needs two things: low temperature and pressure. CO2 dissolves much better in cold water than hot. At a temperature of about 45°F (about 8°C) that most soda makers recommend, 2.2 pints (1 liter) of water can absorb about 0.1 ounces (3 grams) of CO2. At a typical room temperature of 60°F (about 15°C), that falls to just over 0.07 ounces (about 2 grams). Pressure is the other factor. The higher the pressure of the CO2 gas, the more quickly and completely it will dissolve into the water. So, to carbonate water, you chill it and then apply high-pressure CO2.

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Soda makers use a tube or wand that pokes into the water when they carbonate it. The CO2 dissolves into the water on its surface, and creating bubbles increases this area and helps more CO2 dissolve. Look closely when you make fizzy water. You can see some of the small bubbles disappear completely before they hit the surface because all of the CO2 that forms the bubble has been dissolved.

After a little time, the water will have absorbed as much CO2 as it can. As long as there is enough pressure in the CO2 gas above the water, the dissolved CO2 can't escape. Chemists call this an equilibrium: The pressure of the CO2 gas stops the CO2 dissolved in the water from escaping, and the amount of CO2 dissolved in the water stops the gas from dissolving into the water.

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Colin McDonald/CNET

Although the amount of CO2 that can be dissolved in water decreases as the temperature rises, this equilibrium will still hold. Chemists call this a supersaturated solution: The water is holding onto more CO2 than it would absorb at that temperature. It has nowhere to go until you open the bottle, or the pressure of the gas breaks or bursts the bottle. Plastic bottles and metal cans are incredibly strong, but they do burst. You'll see this phenomenon if you leave a can of Coke in a hot car for a long time.

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Colin McDonald/CNET

One odd quirk of carbonation is what happens if you freeze a carbonated drink: The bottle or can usually bursts. Given that cold water holds more CO2 than warm, you might expect the opposite to happen. But cold water and ice are not the same thing, and CO2 is not soluble in ice. When you freeze a bottle of soda, the water freezes and forces out the CO2. This creates a huge amount of gas pressure inside the can. Eventually, the combination of this pressure and the expansion of the ice (which is less dense than water) will burst the bottle or can. That's why you don't freeze sodas.

It also explains the effectiveness of giving someone a can of soda that has been in the freezer for a bit so it gushes out when they open it. The near-freezing soda is pushing the CO2 out, which creates the pressure for the prank to work.

When you open a can or bottle of soda, you break the equilibrium. The gas rushes out and reduces the pressure on the water surface. Suddenly, the CO2 dissolved in the water has somewhere to go, so it starts to escape. It doesn't just rush out of the top, though. Small bubbles form that grow larger as they rise. That's because these bubbles are small surfaces in the water, and more of the CO2 rushes in as they rise.

These bubbles don't just form anywhere, though. They usually start on the surface of the glass, bottle or can that the drink is in because tiny imperfections in the surface form a spot for the minute starter bubbles to form. That's why you see streams of bubbles rising up: Bubbles form on these imperfections until they are large enough that they break off and rise up, and a new bubble forms on the imperfection, and so on.

This is also why the party trick of creating a soda fountain by dropping a mint into a bottle works, because the surface of the mint is covered in imperfections, which creates a sudden rush of bubbles and a fountain of soda.

All about the acid

Carbonation is not just about bubbles, though. The process also changes the taste of the water by creating a sharp, tangy flavor that can complement some drinks. What you may not realize is that this is caused by an acid. When the CO2 dissolves in the water, some of it reacts with the water (with a chemical formula of H20) to form carbonic acid (chemical formula H2CO3). This is a fairly weak acid, but it is an important part of the process because it gives fizzy water the bite that some find appealing. Carbonic acid also has a mild antibiotic effect that prevents bacteria from growing in the water.

Another interesting chemical aside: Until recently, scientists thought that carbonic acid could not exist on its own outside of water. They thought that, without the water that it's normally dissolved in, it would immediately break down. But in 2011, scientists managed to isolate carbonic acid and create stable solid and gaseous carbonic acid for the first time. It's amazing to think that in every sip of fizzy water there's a substance that scientists didn't isolate until this decade. Sometimes, even the mundane and everyday aspects of appliance science can contain surprises...