Instant Ice Crystallization

Hot ice from supersaturated sodium acetate

Difficulty: Medium | Time: 30-45 minutes | Visual Impact: Very High

Historical Context

Supersaturation has fascinated scientists since the 18th century. Gay-Lussac and others studied the phenomenon, trying to understand how a solution could remain liquid when it “should” crystallize. The explanation involves nucleation - the formation of the first tiny crystal seed that triggers cascade crystallization.

Sodium acetate became the classic demonstration of supersaturation because it’s safe, non-toxic, and produces dramatic results. The commercial application came in the 1970s with reusable hand warmers. Clicking a metal disc creates local stress that nucleates crystallization, releasing the stored heat of fusion.

The nickname “hot ice” perfectly captures the paradox: the crystallizing liquid looks exactly like freezing water, but releases heat rather than absorbing it. It’s actually warming while appearing to freeze.

Materials

  • Sodium acetate trihydrate - 100-200g
  • Distilled water - 50mL
  • Pot or microwave-safe container
  • Clean glass dish or bowl
  • Thermometer (optional)

Alternative - Make Your Own: - Baking soda - 84g (1 mol) - White vinegar - enough to neutralize - Pot for boiling

Procedure (with sodium acetate)

  1. Add 100g sodium acetate to 50mL water in a pot
  2. Heat while stirring until completely dissolved
  3. Add more sodium acetate if it all dissolves easily (you want excess)
  4. Once fully dissolved, remove from heat
  5. Pour into a very clean glass container
  6. Cover and let cool to room temperature (or refrigerate)
  7. The solution should remain liquid despite being supersaturated
  8. To trigger: drop in a single crystal of sodium acetate, or touch the surface with a crystal
  9. Watch the entire solution crystallize instantly from the nucleation point!

The Pour Trick

  1. Prepare supersaturated solution as above
  2. Place a seed crystal on a clean plate
  3. Slowly pour the supersaturated liquid onto the seed crystal
  4. A tower of crystals builds up as you pour!

Reaction

\[\ce{CH3COONa(supersaturated) -> CH3COONa(s) + heat}\]

The Science

Supersaturation occurs when a solution contains more dissolved solute than it normally could at that temperature. The solution is metastable - it wants to crystallize but lacks a nucleation site to start. Once triggered:

  • Crystallization releases the heat of fusion (exothermic)
  • Temperature rises to ~54°C (130°F)
  • The process is reversible: heating redissolves the crystals
  • Each crystallization releases ~264 kJ/kg

Tips for Success

  • Use distilled water and very clean containers (impurities cause premature crystallization)
  • If it crystallizes while cooling, reheat and try again with cleaner equipment
  • Save some crystals to use as seed for triggering
  • Can be reused indefinitely by reheating

Resources