Instant Ice Cream: Freezing-Point Depression

Use salt to push an ice bath below 0 °C and freeze cream into smooth ice cream in minutes — a tasty demonstration of a colligative property
Beginner🕐20 min●Low hazardthermochemistryphysical-properties

Summary

Plain ice sits at 0 °C — not cold enough to freeze cream quickly. Add salt and the same ice bath plunges to −18 °C or below, cold enough to turn a cream mixture into ice cream in minutes. This is freezing-point depression, a colligative property that depends on how many particles you dissolve, not what they are. You’ll make (and eat) the result while measuring the temperature drop that makes it possible.

History

Salt-and-ice freezing mixtures were the only way to make ice cream for centuries — long before mechanical refrigeration, confectioners packed churns in salted ice. The same chemistry salts winter roads (lowering the temperature at which the ice/water film refreezes) and is why the ocean stays liquid below 0 °C.

Hazards & preparation

Note

Completely food-safe. The salty ice bath gets very cold — don’t hold it bare-handed for long, and keep the salty melt out of the ice cream. See the Safety page.

Part A — Make the Ice Cream

Materials: ½ cup cream (or half-and-half), 1 tbsp sugar, a splash of vanilla; a small zip bag and a large zip bag; ice; ~½ cup table salt; a thermometer.

Steps:

  1. Mix cream, sugar, and vanilla in the small bag and seal out the air.
  2. Fill the large bag about half with ice and stir in the salt. Slip a thermometer in and watch it drop well below 0 °C.
  3. Nestle the small bag into the salted ice, seal the large bag, and shake/knead for 5–10 minutes (wrap it in a towel — it’s cold!).
  4. The cream mixture freezes to soft ice cream. Wipe salt off the small bag before opening.

Part B — Measure the Depression

Steps:

  1. Put a thermometer in a cup of plain ice water and record the temperature (~0 °C).
  2. Stir a few spoons of salt into another cup of ice and record again.
  3. Try more salt and note how much colder it gets — up to a limit around −21 °C.

What you should see

Plain ice holds near 0 °C; the moment you stir salt in, the thermometer dives to roughly −15 to −20 °C. In the salted bath the cream freezes in minutes, and because constant shaking keeps the ice crystals tiny, the texture comes out smooth rather than grainy.

Symptom Likely cause Fix
Bath won’t go below 0 °C Not enough salt, or not stirred Add more salt and stir; use plenty of ice
Ice cream stays liquid Bath too warm, or not shaken enough More salt; keep shaking the full 10 min
Ice cream tastes salty Salty melt leaked into the inner bag Seal the inner bag well; wipe before opening
Grainy, icy texture Froze too slowly / too still Shake constantly for smaller crystals

The Science

Dissolving salt in the film of liquid water on the ice lowers the temperature at which that water can refreeze — its freezing point drops. Melting ice absorbs heat, and with the freezing point depressed the mixture keeps melting and pulling heat from its surroundings, so the whole bath falls far below 0 °C (down to about −21 °C at salt saturation, the eutectic).

Freezing-point depression is colligative: the size of the drop depends on the number of dissolved particles, not their identity. That’s why salt, which splits into two ions (Na⁺ and Cl⁻) per formula unit, is so effective. The very cold bath then draws heat out of the cream fast; rapid freezing plus constant agitation keeps ice crystals small, giving smooth ice cream instead of a coarse block.

Questions to Explore

  1. Why does adding salt make ice colder, not just melt it?

    Hint / answer

    Salt lowers the freezing point of the water film, so the ice keeps melting — and melting absorbs heat. With nowhere for that heat to come from but the mixture itself, the whole bath cools well below 0 °C.

  2. Why is salt more effective than the same amount of sugar?

    Hint / answer

    Freezing-point depression counts dissolved particles. Salt dissociates into two ions per unit (Na⁺ + Cl⁻), so it produces more particles than sugar, which dissolves as single molecules — a bigger depression.

  3. Why does constant shaking give smoother ice cream?

    Hint / answer

    Agitation breaks up growing ice crystals and freezes the mixture fast, so many tiny crystals form instead of a few large ones — and small crystals feel smooth on the tongue.

Going further

  • Find the limit. Keep adding salt and record the coldest temperature you can reach; compare with the ~−21 °C eutectic.
  • Other solutes. Compare table salt, sugar, and calcium chloride at equal masses and rank the temperature drop.
  • In the Food Chemistry track: contrast this cold physics with the heat chemistry of caramel and honeycomb.