Helena asked: “Is there any difference between seltzer and club soda?”
One of the cool things about water—in addition to how it’s refreshing and delicious, and how life on Earth would never exist without it—is that it’s a powerful solvent, and indeed almost always has stuff dissolved in it.
All those bottles of water on supermarket shelves differ from each other mostly in terms of what’s dissolved in the H2O.
Sparkling water, for instance, has carbon dioxide gas dissolved into it—several grams per liter if you weigh it before and after carbonation—which gives it the compelling bite and tang that we effervescence lovers love. (When it enters your mouth, the dissolved gas diffuses through the skin of your tongue, stimulating the same nerves that horseradish does.)
Like other treats, sparkling water was first discovered occurring in nature, bubbling from underground springs where volcanic activity introduces CO2 into the water under pressure. Popular for bathing since prehistory, people started bottling and distributing the waters centuries ago, and the famous water called Selterser, from fizzy springs in the town of Selters, near Frankfurt, lent its name to seltzer.
“Seltzer” is now a generic term for carbonated water; so is “soda water” and “club soda,” (the latter was originally a trademarked Irish brand). There’s no regulation about the differences between them, but in practice, seltzer- is usually made from plain water that’s been carbonated whereas club soda is made with added dissolved ingredients like salt, sodium bicarbonate, and potassium sulfate inspired by the naturally dissolved minerals in water from springs. Why? For the taste.
Those small amounts of minerals round out the flavor of a water in various ways, from the subtle softness that calcium gives a Schweppes to the salt- and sulfur-tinged bite of Mineragua.
Of the other fizzy waters on the shelf, the ones that come from verified underground sources are labeled “mineral water,” “well water,” or “spring water.” Any minerals and carbonation they contain must come from the same source—although, often, the carbon dioxide and the water are drawn out separately and then combined using standard carbonation techniques in a processing plant. Perrier is an example of this.
The only water that doesn’t contain any dissolved material is distilled water: The distillation process leaves all the minerals behind. It is unpleasantly harsh to drink on its own, but makes great tea, cocktails, and broths, because it’s so thirsty to dissolve anything it comes in contact with.