Carbonated Water and Sodas

Carbonated water is simply water (H₂O) that has carbon dioxide dissolved throughout. Types of carbonated water include sparkling water and seltzer.

What is soda? Sodas (which some people call "pop") are made from carbonated water, but also contain flavors and sweeteners (e.g., sugars; aspartame). One 2 liter bottle of soda includes about 10 grams of CO₂. This (10 g) might not sound like a lot. But what is the volume of 10 g of CO₂?

To find out the volume of 10 g of CO₂, we can use the formula for density:

D = m / V (density is equal to mass divided by volume; see Density for more on density)
V = m / D

At standard temperature (0°C) and standard pressure (1 atmosphere, which is air pressure at sea level), the density of CO₂ is 0.002 grams/mL.

So,

V = (10 g)/(.002 g/mL) = 5,000 mL = 5 L

This means that there are about 5 liters of CO₂ (at standard temperature and pressure) squeezed into and trapped in a bottle of soda! (Sounds crazy, right!?) The volume of CO₂ decreases as it cools and is pressurized, though, which is why it's able to fit in a bottle.

How to make carbonated water. As you may know, if you fill a glass with pure water and set it on a table, the water does not get carbonated on its own. This is because at temperatures around room temperature (about 22°C/71°F), CO₂ molecules that make up the air are moving around fast. Even though H₂O molecules have positive and negative charges, CO₂ molecules are moving fast and have enough kinetic energy to not get "caught" by these electric charges. So, CO₂ molecules stay in the air. Even if a CO₂ molecule happens to break through the surface of the water, it will not stay in the water for long. It will go back into the atmosphere.

In order to carbonate water, it is necessary to reduce the motion of CO₂ and H₂O molecules. In other words, it is necessary to reduce the temperatures of the gas and water. This will allow the H₂O molecules to hold CO₂ in the liquid.

You can even make carbonated water at home (if you have the right equipment, including a tank with high-pressure CO₂ in it!):

First, cool a bottle of water to just above freezing (0°C).
Then, with the cap off of the bottle, squeeze the bottle a bit until the water rises to the top of the bottle (and there is little remaining air in the bottle). The bottle will be a bit "crumpled," with the sides of the bottle pushed in.
Put a cap on the bottle. Use a special cap that has a nozzle on it, so you can pump CO₂ from your tank into the bottle (and not let other air into the bottle).
Pump CO₂ from the high-pressure tank into the bottle. Gently shake the bottle. This will help CO₂ mix with and dissolve in the water.
Repeat the above step until the bottle is no longer crumpled. It is now pressurized carbonated water! Enjoy!

The CO₂ molecules are now trapped in the water. Because the water is cooler (and H₂O molecules are moving more slowly), the water is a bit more dense and the bonds between H₂O molecules are also a bit stronger. The CO₂ molecules themselves also have less kinetic energy, or energy due to motion. So, they are less able to break through the (stronger) bonds between H₂O molecules and leave the liquid.

As shown in the picture above, H₂O molecules form spheres around CO₂ molecules (carbon atoms in the CO₂ are represented as larger and darker blue spheres and oxygen atoms as white spheres). This bonding of H₂O molecules to other H₂O molecules helps to hold the CO₂ molecules in the liquid.

Another reason that CO₂ remains dissolved in the water is that there is air at the top of the bottle. This is high-pressured CO₂, where there are a lot of CO₂ molecules per volume. This high-pressure gas pushes down on the surface of the liquid. This pressure also helps prevent CO₂ molecules in the liquid from leaving the liquid.

When you open a bottle of soda, you'll hear a "whooshing" sound. That's the sound of the high-pressure air leaving the bottle. You probably also noticed that some of the soda may bubble up (and out of the bottle, if you're not careful). This is from some of the CO₂ in the soda leaving the soda. Since there is now less air pressure at the surface of the soda, it's easier for the trapped CO₂ near the surface to escape from the soda.

If you leave an open bottle of soda sitting around (with no cap on it), eventually it will lose all of its carbonation. (It also does not taste very good.) As the soda's temperature increases, the H₂O molecules begin moving faster and the bonds between H₂O molecules weaken. The CO₂ molecules also start moving around faster, enabling them to break through the H₂O bonds. These factors allow CO₂ to eventually escape from the soda.

Soda "Explosion." As we discussed at the beginning of this unit, a 2 liter bottle of soda has about 5 liters of CO₂ (at room temperature) trapped in it. This is 2.5 times its volume! When the water temperature is low, the water molecules help to hold the CO₂ molecules in the water by H₂O-H₂O bonds. But when enough these bonds are broken, the CO₂ molecules can "break free" from within the water and cause the soda to "explode." This may happen when you shake a bottle of soda before opening it. The motion of shaking the soda may help break H₂O-H₂O bonds and allow the CO₂ molecules to escape from the water.

CO₂ is less dense than water. Because of this, CO₂ bubbles will be pushed upward by buoyancy force (see Buoyancy force unit), causing it to escape from the water.
The CO₂ in soda is also at a higher pressure than the air outside the bottle (it had to be forced into the bottle). This difference in pressure — once enough CO₂ molecules are freely moving around in the soda — will also push the CO₂out of the bottle.

If enough CO₂ is able to escape (remember, there is a LOT of CO₂ in the water: around 5 liters!), it can push a lot of the liquid soda out of the bottle and cause a big "explosion" or "geyser."

One cause of CO₂ exploding from a bottle of soda is "nucleation", where bubbles form on tiny bumps on surfaces (see Nucleation unit for more information).