Way back in 1974/75, as a fresh high school graduate, I worked at a pop bottling plant. All comercially made soda-pop is made by combining sugar, water, and flavoing syrup together to form a thick syrup. Depending on the bottle/can size, a precise amount of this syrup is added to the container, followed by carbonated water. The container is sealed and shaken to distribute the syrup. The result is pop.
In the plant where I worked, the water was carbonated by passing filtered water through pressure chambers containing blocks of dry ice (frozen Carbon Dioxide). The water became carbonated as the carbon dioxide dissolved in the water under pressure.
Home-brewed rootbear is carbonated by the action of yeast organizms eating some of the sugars. As was stated above, this results in the fromation of both alcohol and carbon dioxide gas. As the gas is trapped, it dissolves into the water and creates a carbonated beverage, with alcohol in it as well. The same process turnes grapes into champain, and apple juice into apple jack.
Home made pop can also be made by adding very small amounts of dry ice to watered down and sweetened juices, or even to kool-aid type beverages. I did this as an experiment with one of my children. We washed out a 2 liter pop bottle, and its cap. We added grape juice, which had been watered down and sweetened until it tasted like a strong kool-aid. The we added jsut a sliver of dry ice and put the cap on.
As dry ice is much more dense than it is in its gasseous form, it caused the plastic bottle to expand quite dramaticaly at first. We were afraid the bottle would explode. So I let some of the gas pressure out by gently opening the cap until it escaped. The we put it in the fridge. My daughter was very impressed with the home-made pop. Of course she was a very young teen at the time.
Hope this helps and also gives you ideas on how to make your own carbonated beverages.
Oh, and the root bear flavor can be had by making a strong wintergreen tea, sweetened with brown sugar. Birch buds will also give you this flavor when made into a tea, as will sasafrass root. If none of these are available to you, then you can usualy find root beer flavoring or extract at the grocers.
And finally, you can just purchase sparkling water and add the flavoring syrup to it to make your own pop.
And as for the poster who said that he/she prefers the more natural cabon dioxide from yeast, I will categorically state that the gas is exactly the same whether it comes from biological activity (yeast), or from evaporating dry ice. There just is no difference in the gas. But with the yeast process, you get alcohol along with the carbon dioxide.
Seeeeeya; Goodweed of the North