The Scotch & Soda is two ingredients and a glass of ice. Blended Scotch, cold club soda, maybe a lemon twist if you want the bartender to think you've thought about it. It's the drink that taught a generation of Americans what Scotch was for, and it's still the best use of a mid-shelf blend I know.
Soda doesn't hide a blended Scotch. It opens it up, which is exactly what the blenders built it to do.
History
Whisky and soda was a British habit before it was an American one. Bottled soda water was a commercial product in Britain by the early 1800s, and blended Scotch, lighter and more consistent than the single malts it was built from, became the export style the big houses sold abroad in the late nineteenth century. Put the two together and you have a drink that travels.
In the US it settled into the hotel-bar highball tradition of the 1890s, covered in more detail on the Whiskey Highball page. Nobody invented it. There's no single bartender, bar, or year to credit, and anybody who tells you otherwise is selling a bottle. The Kingston Trio put "Scotch and Soda" on their 1958 debut album, which tells you how ordinary the order had become by then.
The Spec
Two ounces of blended Scotch into a highball glass full of ice, four ounces of cold club soda down the side, one gentle lift with the bar spoon. That's a 1:2 build, a little stronger than the Whiskey Highball's 1:3 because Scotch carries less sugar than bourbon and can take it.
Which Scotch
A blend. Dewar's White Label, Johnnie Walker Black, Monkey Shoulder, Famous Grouse. Blends are made to be lengthened, and soda flatters their softer malt-and-grain body. A heavily peated Islay malt works if smoke is what you want, but it takes over the glass. Save your 18-year single malt for a glass with no soda in it.
Soda and Ice
Freshly opened, very cold, very fizzy. A half-flat bottle from the back of the fridge turns this into weak whisky water. Fill the glass with ice to the top so the soda stays cold and loses less gas on the way in.
Bottom Line
Order it when you want a drink that tastes like the whisky you paid for, only longer. Two ingredients. Hard to ruin. Not impossible.