Written by Rabbis Eli Gersten, Yaakov Luban, and Moshe Zywica of the Orthodox Union
Seltzer is made by injecting carbon dioxide into water. When this is done, the water immediately begins bubbling. Are these bubbles included in the prohibition of molid (creating something new)?
The Ben Ish Chai (Rav Poalim OC 1:18) writes that one may make seltzer on Shabbat. He writes that since one does not actively make the water bubble, but rather by adding the carbonating ingredients to the water it will begin to bubble on its own, it is permitted. This is comparable to melting ice by placing it into water. Although one may not rub ice to make it melt, one may allow ice to melt on its own by placing it into water.
Other poskim such as Chelkat Yaakov (OC 134) are strict based on “uvda d’chol” (the actions has the appearance of a melacha), but the more common custom is to allow. This was also the ruling of Rav Ovadya Yosef zt”l (Yebia Omer OC 3:21), Shemirat Shabbos K’hilchata (11:36), and many other poskim.