Colligative

set the ice cream. the cold counts pieces.

an old ice cream bucket: throw scoops of sugar, salt or calcium chloride on the ice to land the brine in the band that sets the cream; each cools it by the pieces it splits into

Anything dissolved in water lowers the temperature at which it freezes, by how many separate pieces it makes, not by what they are or how much they weigh: dT = i x Kf x m, with Kf = 1.86 C kg/mol for water. Every scoop here holds the same number of molecules, half a mole for a kilo of meltwater (171 g of sugar, 29 g of salt, 55 g of calcium chloride), so the big heap of sugar stays 1 piece each and cools the brine 0.93 C, salt falls apart into 2 ions (1.86 C) and calcium chloride into 3 (2.79 C). The dial is that ideal rule; real strong brines stray from it, and salt with ice bottoms out near -21 C. The wedge is the game's: in a real churn too little salt never sets the cream and too much freezes it fast and coarse.

Concepts: chemistry, phase-change.

An interactive you play with one thumb, on Wundr.