They all sparkle. They all bubble. And they’re all water. But what's the difference between each one?
Club Soda: This is just water with a few add-ins: carbon dioxide for carbonation and “mineral-like” ingredients such as sodium bicarbonate, sodium citrate, and potassium sulfate. It’s unflavored, other than the slightly mineral-y taste.
According to Culinary Lore, Club Soda (note the proper noun) was - and still is - trademarked in Great Britain and Ireland by Dublin-based beverage company Cantrell & Cochrane. However, in 1896, the company lost a lawsuit against a Jamaican company that had used the term club soda, which is probably why it remains a generic term everywhere else.
Seltzer: Once again, this is also just plain water with carbon dioxide added for carbonation, but without the mineral additions. Unlike club soda, seltzer can be sweetened and flavored, often with citrus or other fruits. Seltzer was never trademarked as a term, so any soda water brand can use it on their labels and there are not legal specifications as to what it should contain, or not contain, other than carbonated water.
Sparkling Water: Sparkling mineral water usually comes from a natural spring, which may provide natural carbonation. There’s also sparkling water that isn’t mineral based and doesn’t come from a spring; it's simply carbonated water, sometimes flavored.
Seltzer, club soda, soda water, or just soda, are all used interchangeably in common usage. But now you know what the subtle differences are!
Now you may be thinking... but what about tonic water? Tonic water is also - here we go again - carbonated water. The big difference is the addition of quinine, an ingredient found in the bark of the South American cinchona tree. Quinine was originally used as malaria medicine. According to legend, British soldiers started mixing the bitter stuff with soda water, sugar, and gin, to make it go down easier.