Commonsense would tell you that adding water to a drink would dilute its flavour: we even have a figure of speech, watered down, to mean something has been made more bland, less interesting, than it was originally. It wouldn't be unreasonable, then, for you to be perplexed by the classic move often made by seasoned whiskey drinkers of adding a dash of water to their tumbler.
By those who choose to do so, the argument is made that a few drops of water, rather than toning down the whiskey, in fact elevate the flavour of the liquor; opening up new and subtle characters within the taste and aroma that can't be discerned when it's imbibed as a straight pour from the bottle.
Well as of earlier this summer, chemists have stepped forward to give us the final answer on just what a little water does to your whiskey. The answer is all in the molecules. Or rather, all in how they interact at the "liquid-air interface". Scientists at Linnaeus University in Sweden modeled the molecular motion of ethanol (the alcohol in whiskey) when interacting with water using a computer simulation. They then added single molecules of guaiacol, the component in whiskey responsible for its sweet and smoky flavours, which comes from the wooden casks the spirit in matured in.
The simulations showed how guaiacol molecules bind to alcohol molecules, and in turn partially repel water. At lower concentrations of alcohol, these taste-creating molecules cluster at the top of the liquid the whiskey glass. In comparison, when the drink is actually stronger – when less water is added to the alcohol – the density of guaiacol on the surface is lower, and the taste of the drink is less pleasing.
This change to the flavour profile is the reason why whiskey as you buy it has already been diluted with water. The typical bottle – whether scotch, Irish, bourbon or rye – is around 40% alcohol by volume. Straight from the cask, whiskey can be up to 65% ABV.
All of which explains why, for the most aromatic and flavourful sip, adding a little extra water to your whiskey is a good idea: it will only make your nose and mouth experience the smoky, bitter and sweet tastes as more full-bodied. Be warned, though. If you dilute your drink too much, after a certain point the tasty guaiacol molecules will become undetectable to your tongue!
Our advice? Trust your own senses, and figure out how you enjoy your whiskey best.