Every old recipe for homemade ice cream says the same strange thing: surround the can with ice, then pour in salt. Salt is not cold. The ice is already cold. So why does adding salt make the bucket colder, cold enough to freeze cream?
At exactly 0 °C, pure ice and pure liquid water sit in balance. Water molecules are leaving the ice surface and joining it at the same rate. Melting and freezing cancel out.
Now dissolve salt in the water. Sodium and chloride ions get in the way of water molecules trying to rejoin the ice, but they do not interfere with molecules leaving it. Melting carries on, freezing slows down, and the balance tips toward liquid. To rebalance, the mixture has to get colder. That drop in freezing point is called freezing point depression, and it depends on how many dissolved particles there are, not what they are. Salt splits into two ions per unit, so it works especially well.
Melting ice needs energy, about 334 joules for every gram. With salt in the way, the ice keeps melting even below 0 °C, and it takes that heat from the only place it can: the brine itself, and anything touching it, like your cream. The bucket cools until it reaches the new, lower freezing point of the brine that is forming. Then it stops. The salty slush is stuck at exactly that temperature.
Drag the slider to add salt to the ice bucket. The can holds ice cream mix, which freezes at around −2 °C because of its own sugar and solids.
You cannot keep adding salt for ever. Salt water reaches its coldest at about 23.3% salt by weight, where it freezes at roughly −21 °C. This is the eutectic point. Add more and the extra salt just sits undissolved, because the water can hold no more at that temperature. That is why the curve turns into a flat line, and why a bucket of ice and salt never goes colder than about −21 °C however much you pour in.
Real ice cream buckets are not at the eutectic. Recipes use a modest amount of salt, a lot less than the 23% maximum, and the brine ends up several degrees below zero. That is plenty to freeze cream quickly, but not so cold that the mix freezes solid against the can wall before the paddle can scrape it. The cranking matters too, because it keeps ice crystals small and the texture smooth.
Road crews use it in reverse. Spread salt on an icy road and the ice-water film on top gets salty, so it can stay liquid below 0 °C. It works down to about −9 °C in practice; below that, there is too little melting to matter, and crews switch to other chemicals or sand. The same physics explains why seawater, with about 3.5% salt, freezes near −1.9 °C.
So the next time you see salt poured on ice, remember: it did not add cold. It just stopped the ice from being able to stay in balance until the whole bucket gave up its heat.