What is the difference between soda water, tonic, seltzer and sparkling mineral water?
What is dissolved in them besides carbon dioxide — ranging from nothing at all to a substantial amount of sugar.
Seltzer. Plain water that has been carbonated, with nothing added. The name derives from Selters in Germany, originally a naturally sparkling spring, though modern seltzer is artificially carbonated plain water.
Soda water (club soda). Carbonated water with added mineral salts — typically sodium bicarbonate, potassium sulphate or similar. These give it a very slightly saline, rounder taste compared with seltzer's neutrality. The additions were originally intended to imitate natural mineral springs.
Sparkling mineral water. Water from a protected natural source, containing naturally occurring dissolved minerals whose composition must be stable. In the EU and UK, "natural mineral water" is a legally defined term with strict rules: it must come from a specific identified source, be bottled at source, and may not be treated in ways that alter its mineral composition.
It may be naturally carbonated, carbonated from the spring's own gas, or artificially carbonated — and the label must say which. The mineral content varies enormously between sources, which is why they taste noticeably different.
Tonic water. The one that is genuinely different: carbonated water containing quinine and, in almost all cases, a substantial amount of sugar.
Quinine, from cinchona bark, is what makes it bitter. Its origin is medicinal — it was used against malaria, and British colonial officers mixed it with gin and sugar to make the bitter dose palatable, which is where the gin and tonic comes from. Modern tonic contains far too little quinine to have any medicinal effect, and the amount permitted is regulated.
Tonic is not a mixer in the same category as the others. A standard bottle contains a comparable amount of sugar to many soft drinks, so a gin and tonic is considerably more caloric than a gin and soda. Slimline versions use sweeteners.
Quinine fluoresces under ultraviolet light, which is why tonic water glows blue under a UV lamp.
Sparkling water does not damage tooth enamel significantly, being far less acidic than soft drinks, though flavoured versions containing citric acid are more erosive.