You'd think that when dealing with something as simple and straightforward as water, there couldn't be any confusion. Water is water, right? Confusingly enough, this isn't so. Living in a time when information is accessible through small portable devices that we keep in our pockets, it's only natural that things just cease being simple -- they become tampered, enhanced and they require explanation. And water is not exempt from that.
You may have noticed when walking down the aisle of the grocery store that there are many options when it comes to carbonated water (also referred to as bubbly water, sparkling water, club soda, soda water and seltzer). And despite what you might think, this isn't just a difference of regional terminology; it isn't the Europeans calling it sparkling, the New Englanders seltzer and everyone else club soda. And then there's also tonic water -- what's that, anyway?
There's a real, bonafide difference between these types of water. And while the discrepancies are somewhat slight, they're enough of one for the waters to merit their own name, and so it's worth getting to the bottom of. Here's what you need to know:
Sparkling mineral water comes from a natural spring which contains various minerals, like salts and sulfur compounds. It's defined by its "constant level and relative proportions of mineral and trace elements at the point of emergence from the source."
Minerals aren't added to this water and neither is carbonation (with the exception of San Pellegrino, which has additional carbonation added by the bottler). That means that the bubbles in these bottles are completely natural. You would typically drink this water as is (not mixed in a cocktail), since it's a tad expensive and has a slight mineral-y taste.
Seltzer water is just plain water that has been artificially carbonated. This water, which contains no sodium salts, gets its name from the German town of Selters, which was renowned for its natural springs. Seltzer water was first introduced as a cheap alternative to sparkling mineral water -- and it still is an economical option today.
Seltzer water and club soda are very similar, but there is a notable difference between the two. Unlike seltzer, mineral-like ingredients are added to club soda to enhance the flavor. If you look on the list of ingredients, you'll likely see potassium bicarbonate and potassium sulfate listed. Regardless, you could still swap one for the other without really being able to pick up on a difference of taste (though according to some, it can be detected).
Just because it contains the word water in its name and is carbonated doesn't mean that tonic water is in the same category as bubbly waters. Unlike the other carbonated options, tonic water has a distinct flavor and it certainly can't be swapped out (or in) for carbonated water. Tonic water is a bitter drink (a result of the addition of quinine) which pairs particularly well with gin. Also unlike the other waters, Tonic contains calories -- about 130 for 12 fluid ounces.
CORRECTION: The post initially suggested that San Pellegrino does not contain additional carbonation; it does.