Soda has something in it that almost no other liquid has: lots of carbon dioxide. The carbon dioxide or CO2 is a gas that gives soda its fizz. There are a few places in the world where water absorbs CO2 by flowing over rocks and minerals with certain chemicals in them. The sodas we buy have been carbonated at the soda factory.
Carbon dioxide is made of 1 carbon and 2 oxygen atoms that form a CO2 molecule. Plants need this gas for making food and we exhale this gas. It's a heavier gas than our air and will cause a balloon to sink instead of float like a helium balloon.
The soda we drink is mostly water (H2O) 2 hydrogen and 1 oxygen atoms.
They add sweetner, flavoring and CO2. It's not easy to put CO2 in the water. It's forced in with a lot of pressure and sealed right away. When you open a soda you can hear the CO2 escape. They use very cold water too. Cold water holds more CO2 than warm water does. Let me know what flavors you come up with