When Scotch is taken directly from the cask, it’s at an alcohol by volume percentage known as “cask strength” or “barrel proof.” There is no standard for how strong “cask strength” is but most scotches are diluted down to around 40% ABV to 43% ABV. For those that don’t, usually older scotches where buyers want to have it at cask strength because it represents the product straight from the barrel, cask strength usually means it’s in the 60’s in terms of percentage alcohol by volume.
At cask strength, the alcohol really dominates most of the other flavors; which is why many enthusiasts will dilute the product down to a more typical 40%-43% ABV. By adding water, you also have a chemical effect on the whiskey; you break down ester chains and release some of the more volatile aromatic elements of the scotch. Ever notice that you’ll smell things you didn’t smell before when you’ve sipped the whiskey? That’s because the saliva, and other goodies, in your mouth also break down some of those ester chains.
How should you dilute the scotch? The best method is with distilled water, especially if you have particularly hard water (heavy in minerals). You want your water to be as flavorless as possible and distilled water is the best choice, second only to water from the distillery! How much should you add? Not too much if you’re drinking something already at 40%, maybe a dash to release some flavors.
Or… add none. 🙂