Tonic water is a carbonated soft drink with a very bitter flavor. It is made from water that is mixed with a variety of minerals and carbonated. Tonic water has its sour taste because of quinine, a compound found in the bark of cinchona trees in South America and Central America. Because of its bitter flavor, tonic water is commonly used in cocktails including gin and tonics.
Like tonic water, club soda is also a carbonated drink with a water base. However, quinine isn't a part of the equation of this fizzy drink. Club soda is carbonated water that is infused with minerals and salt to replicate the texture of soda without all the added sugars and preservatives. Also known as carbonated water, seltzer water or sparkling water, club soda is sugar-free, typically unflavored and has zero calories. It's become popular in the mixology world and is commonly used in many cocktails, typically in those containing gin or vodka.
Tonic water and club soda share the same base of highly carbonated water, and each drink includes various minerals and compounds. Both are also common ingredients in popular cocktails, though their uses vary.
Though the two drinks have a similar base and carbonated element, the ingredients make these drinks different. The quinine makes tonic water drastically more bitter than club soda, which, while a bit salty, is relatively bland in comparison. Tonic water also contains sugars or other sweeteners, which can make it high in calories while club soda is always calorie-free. Because of its bitter taste, tonic water is typically never consumed on its own, while club sodas can be.
Choosing which drink to add to your cocktails is up to your own personal preference. If you enjoy a bitter taste, prefer your alcohol full and heavy and don't mind a little extra sugar content in your drinks, you will probably prefer tonic water more than the mild flavor of club soda. However, if you aren't a fan of strong drinks, would like to bring down the intensity of some of your favorite alcoholic beverages, or are simply trying to cut back on your intake of sugar, club soda may be the optimal mixer for your next late night beverage.