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Ask Dr. Knowledge

What is tonic water?

March 1, 2010

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What’s “tonic’’ about tonic water? Also, how is club soda different from mineral water?

Tonic water is carbonated water (more on that in a moment) to which quinine has been added to give it a characteristically bitter taste, which many people - myself among them - quite enjoy.

The original tonic water was actually medicinal, and very bitter, containing large amounts of quinine and no sweetener. Quinine fights and can prevent malaria. It comes from the bark of the South American cinchona tree - the word quinine is from the Quechua (Inca) word for that bark.

When the British Empire was busy taking over tropical parts of the world, tonic water was widely drunk to ward off disease. The now-popular gin and tonic originated in British colonial India, where people took to mixing tonic water with gin to mask the bitterness, though I think most people nowadays use tonic to soften the taste of gin.

Quinine itself is a prescription drug in the United States, with the FDA limiting the amount of quinine in tonic water to about 83 parts per million, which is a mere quarter to a half percent of what a medicinal concentration would be. Don’t count on tonic water to protect you from malaria. In addition, quinine was believed to reduce fever and inflammation and may help some people who get cramps.

Club soda is water with carbon dioxide dissolved in it to make it fizzy.

The English chemist Joseph Priestley invented it by holding a bowl of water over a beer vat. The process of fermentation released carbon dioxide (the bubbles in your beer) and this dissolved in the water, making the first “carbonated water.’’

Nowadays, one just pushes carbon dioxide at high pressure through water, making lots of it dissolve quickly. Small amounts of mineral salts, such as sodium or potassium bicarbonate or table salt, are often added to club soda at the whim of the manufacturer to modify its taste and emulate mineral waters, which are naturally occurring waters with dissolved minerals (and possibly also dissolved carbon dioxide, if they are the “sparkling’’ kind).

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.