Adding water to spirit releases heat and destroys volume — two facts that make every strength reading taken on a freshly reduced tank wrong. Here is the physical chemistry behind it, why the whole industry quotes strength at 20 °C (or 60 °F), how large the gauging error actually is, and what temperature your blending water should arrive at.
A whisky forgives a little calcium because there is nothing in the bottle for calcium to react with. A liqueur is not so lucky — it carries sugar, fruit acids and, in the cream category, a dairy protein emulsion that ionic calcium is known to coagulate. This is why liqueur producers buy demineralised water, and what happens when they do not.
A distillery can specify blending water to a few parts per million and then undo the work in the final ninety seconds, by rinsing the empty bottle with whatever comes out of the mains. Residual rinse water dilutes the spirit in one direction only, and whatever was dissolved in it stays in the bottle after the water has gone. Here is the arithmetic, and why the industry rinses with air or with spirit instead.
A reverse-osmosis membrane rejects 95–99% of dissolved salts, which is why every serious blending-water train starts with one. But rejection is a function of charge and molecular size, and a handful of contaminants have neither — dissolved gases, boric acid, some silica, and the small neutral organics you can actually smell. Here is what passes, which of it matters in a bottle, and what a buyer should ask for beyond an RO spec.
Demineralisation strips a water of everything measurable except the thing forensic laboratories actually read. The hydrogen and oxygen atoms in a water molecule carry a signature of the rainfall that produced them, and in a bottle at 40% ABV those molecules account for most of the oxygen in the liquid. Counterfeit Scotch has been identified on exactly that basis — which makes the blending water a matter of brand protection as well as flavour.
A blending water can pass conductivity, TDS, pH and microbiology and still carry an earthy or antiseptic note straight into the bottle — because the compounds that cause it are active at nanograms per litre, below what routine instruments report. The drinking-water industry solved this with formal sensory methods decades ago. Here is how they work, what a trained panel can detect that a lab cannot, and how a distillery should smell its water before it proofs a cask.