Food · 4 minute read
Every old ice-cream recipe says the same odd thing: pack the bucket with ice, then pour on rock salt. Ice is already as cold as water gets before it melts. So how can a handful of kitchen salt make it colder?
At 0 °C an ice cube is not sitting still. Water molecules at its surface keep leaving the crystal, and other molecules from the liquid keep landing on it. When the two traffic flows match, the ice neither grows nor shrinks, and 0 °C is the temperature where that balance holds for pure water.
Now stir in salt. Sodium and chloride ions dissolve into the thin film of liquid on the ice. The water molecules still leave the ice at the same rate, but the ions crowd the liquid, so fewer water molecules land back on the crystal. Melting wins. This is called freezing-point depression: the more particles dissolved, the lower the temperature must fall before freezing catches up again.
Melting ice takes energy: about 334 joules for every gram. With salt, the ice is suddenly keen to melt at 0 °C, but there is no spare heat to pay for it. So the energy is taken from the nearest source, which is the brine, the metal can and the cream inside it. Everything cools until the brine is cold enough to be in balance with the ice again.
How cold depends on how salty the brine is. Table salt works out to roughly 6.5 °C of cooling at 10% salt by weight, and about 16.5 °C at 20%. The curve stops at a floor: the NaCl–water eutectic, 23.3% salt by mass and −21.1 °C. Add more salt than that and it just sits at the bottom of the bucket. Past that point nothing gets colder.
Drag the orange dot, or use the slider. Salt is shown as % of the brine's weight.
Plain ice at 0 °C can chill cream, but it cannot freeze it. Cream is full of sugar and other dissolved solids, so it freezes a little below 0 °C, roughly around −2 to −3 °C. Plain ice would leave the cream only a hair colder than it needs to be, and almost nothing is left to pull out the heat that freezing releases. A salty brine at −10 °C or lower turns the bucket into a proper freezer.
The same trick is why road crews spread salt. It works the same way, melting ice at temperatures below 0 °C, until the weather gets so cold the brine cannot keep up.
Uses the brine you set in Lab 1. Press churn and watch the firmness bar. This is a simple model, not a stopwatch.
Ice crystals in ice cream are what you feel on your tongue. Big ones feel gritty, small ones feel smooth. Cold from a salty brine freezes the cream at the wall of the tub quickly, and a paddle scrapes that frozen layer off before the crystals grow big. The stirring also whips in air and keeps the colder cream from piling up against the wall. That is why a frozen block that was left to sit feels coarse, while churned cream feels like cream.
Salt does not make ice colder by itself. It makes ice melt at a temperature where it would normally be frozen solid, and melting pulls heat out of everything around it. The result is a bath that can fall to about −21 °C at best, which is cold enough to freeze cream.
Numbers: NaCl–water eutectic 23.3% by mass at −21.1 °C (chemguide.co.uk; Frostburg State University chemistry notes); a 10% brine freezes near −6.5 °C and 20% near −16.5 °C (standard freezing-point tables, consistent with the values quoted by Steve Spangler Science and the US Federal Highway Administration); latent heat of fusion of ice about 334 J/g.