Water

Still water is water. Sparkling mineral water is spring water with carbonation, either its own or added later. Soda water is usually carbonated water with mineral salts stirred in, which gives it a faint saline edge that you can pick up if you taste it on its own. Tonic is a sweetened soft drink with quinine in it and belongs in a different conversation entirely. Only the first three are unsweetened, and the fourth is regularly grouped in with them by mistake at a home bar.

Adding water to a strong whisky is long-standing practice, particularly with cask strength bottlings. The usual explanation is that dilution changes how aroma compounds behave at the surface of the liquid, making more of them available to the nose. The mechanism is argued over more than the practice is. Either way the advice is consistent: add water a few drops at a time, taste between each addition, and stop before the texture goes thin, because you cannot take it back out.

There is no shared scale for how fizzy a sparkling water is, and the gap between a softly carbonated mineral water and a hard-charged soda is wide. Softer carbonation suits drinking on its own and works with delicate spirits. Harder carbonation survives ice and dilution better, so it holds up in a tall drink that is going to be sipped slowly over half an hour.

Carbon dioxide dissolved in water forms a small amount of carbonic acid, so sparkling water is mildly acidic and reads on the tongue as a faint sourness alongside the prickle. That is why it tastes less flat than still water even though neither contains sugar or flavouring. It is also why a spirit topped with soda comes across as drier than the same spirit topped with still water.

Glass is inert, so water that has been in a glass bottle tastes of nothing but itself even after long storage. Plastic is slowly permeable to gas, so a sparkling water in a PET bottle softens on the shelf over months. For a still water sitting in a fridge for a week, none of this makes any difference.