Instant Ice Crystallization

Hot ice from supersaturated sodium acetate — trihydrate and anhydrous routes

Difficulty: Medium | Time: 30–60 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 is 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” captures the paradox: the crystallising liquid looks exactly like freezing water, but releases heat rather than absorbing it.

The Two Forms of Sodium Acetate

This experiment can be done with either form of sodium acetate, but the procedures differ significantly because the two forms have very different water content.

Property Trihydrate (CH₃COONa·3H₂O) Anhydrous (CH₃COONa)
Molar mass 136.08 g/mol 82.03 g/mol
Water content 39.7% by weight 0%
Appearance White crystalline solid, often clumps Fine white powder
Melting point 58°C (melts into its own water) 324°C
Behaviour on heating Dissolves in its own water of crystallisation Requires added water
Extra water needed Very little (0–20 mL per 100 g) ~35–50 mL per 100 g

The crystallisation product is the same in both cases: below 58°C, the stable solid form is always the trihydrate. Whether you start with trihydrate or anhydrous, what crystallises is CH₃COONa·3H₂O.

Procedure A — Sodium Acetate Trihydrate

This is the simpler route. The crystals already contain enough water to dissolve themselves.

Materials: - Sodium acetate trihydrate — 150 g - Distilled water — 10–20 mL (may need none at all) - Small pot or saucepan - Very clean glass bowl or jar - Seed crystals (a few saved dry)

Steps:

  1. Place 150 g trihydrate in the pot with 10 mL distilled water
  2. Heat gently, stirring — the crystals will melt into their own water of crystallisation around 58°C
  3. Continue heating until fully dissolved and the solution is clear
  4. If any crystals remain undissolved, add a few more mL of water and reheat
  5. Remove from heat; pour carefully into a very clean glass bowl without disturbing the bottom of the pot
  6. Cover with cling film and allow to cool undisturbed to room temperature (~30–45 min)
  7. The solution should remain liquid — this is the supersaturated state
  8. To trigger: drop a single dry seed crystal onto the surface, or touch with a crystal on the tip of a spoon
  9. Crystallisation propagates instantly from the nucleation point, and the solid warms noticeably to ~54°C

Why so little water? The trihydrate is 39.7% water by weight — 150 g of trihydrate already contains ~60 g of water. The extra water simply helps ensure complete dissolution without adding excess.

Procedure B — Anhydrous Sodium Acetate

Anhydrous sodium acetate contains no water of crystallisation, so you must add water to dissolve it and then evaporate enough to achieve supersaturation on cooling.

Materials: - Sodium acetate anhydrous — 100 g - Distilled water — 40 mL - Small pot - Very clean glass bowl or jar

Steps:

  1. Add 40 mL distilled water to the pot and bring to a gentle simmer
  2. Add 100 g anhydrous sodium acetate gradually, stirring to dissolve
  3. Heat and stir until fully dissolved — the solution will be clear and quite viscous
  4. Continue heating gently (do not boil vigorously) to evaporate a small amount of excess water — ~5 minutes of gentle simmering
  5. Test for correct concentration: a drop placed on a cold dry surface should solidify quickly and feel warm. If it stays liquid and cool, simmer a few minutes more
  6. Pour carefully into a very clean glass container; cover and cool undisturbed
  7. Trigger as in Procedure A

Why more steps? Anhydrous sodium acetate must be dissolved in water to behave as trihydrate does. The target is to end up with the same ratio: approximately 3 moles of water per mole of sodium acetate (i.e., ~54 g water per 82 g anhydrous ≈ 65% solution by weight). Starting with 40 mL (40 g) of water and evaporating a little gets close to this ratio.

Make Your Own from Baking Soda + Vinegar

Both anhydrous and trihydrate can be made from scratch:

  1. Measure 84 g baking soda (1 mol NaHCO₃) into a pot
  2. Slowly add white vinegar (5% acetic acid), stirring — approximately 1.2 litres needed
  3. The reaction produces sodium acetate, water, and CO₂:

\[\ce{NaHCO3 + CH3COOH -> CH3COONa + H2O + CO2 ^}\]

  1. Once fizzing stops, heat the solution to evaporate water
  2. For trihydrate: evaporate until a thick syrup remains; cool — you will get trihydrate crystals
  3. For anhydrous: continue heating until dry and the powder just starts to turn slightly yellow — stop here. The anhydrous form is stable above 58°C

The Pour Trick

This works with either procedure:

  1. Prepare supersaturated solution and cool to room temperature
  2. Place a single seed crystal or a small pile of crystals on a clean dry plate
  3. Very slowly pour the supersaturated liquid onto the seed
  4. A tower of crystals builds upward as you pour — each droplet crystallises on contact and provides the next nucleation surface

The Science

Supersaturation

At room temperature (~20°C), the solubility of sodium acetate trihydrate is about 36 g per 100 mL of water. The prepared solution contains far more than this — it is supersaturated: the solute is held in solution only because there are no suitable nucleation sites to begin crystallisation. The solution is metastable; any disturbance — a seed crystal, a scratch on the glass, a vibration — provides the surface energy needed to start the cascade.

Heat of Crystallisation

Crystallisation of sodium acetate trihydrate from supersaturated solution is exothermic — energy is released as the ions arrange into an ordered crystal lattice and as the water molecules bind into the hydrate structure. The temperature rises to approximately 54°C and ~264 kJ/kg is released — enough to make a hand warmer comfortable for 30–60 minutes.

\[\ce{CH3COONa(aq, supersaturated) -> CH3COONa.3H2O(s) + heat}\]

Why Trihydrate, Not Anhydrous?

Below 58°C, the thermodynamically stable form of sodium acetate is the trihydrate. The anhydrous form is only stable above this temperature. When a concentrated aqueous solution cools below 58°C and crystallises, it must crystallise as the trihydrate — the water molecules are incorporated into the crystal structure as it forms. This is why anhydrous sodium acetate dissolved in the correct amount of water gives exactly the same crystallisation behaviour as trihydrate does.

Reuse

The crystallised solid can be reheated indefinitely: - Heat slowly until all crystals dissolve (~58°C for trihydrate) - Cool carefully without disturbing - The solution is supersaturated again, ready to trigger

No chemistry is consumed in the process — it is purely a physical phase change cycling between solution and crystal.

Tips for Success

  • Cleanliness is critical: any dust, scratches on the glass, or undissolved particles will trigger premature crystallisation. Use clean glassware and rinse with distilled water
  • If the solution crystallises while cooling, reheat to redissolve and try again — this is not a failure, just impurities triggering nucleation
  • Save a few dry seed crystals before dissolving everything — these are the most reliable trigger
  • Pouring into a glass jug (rather than a wide bowl) makes the pour trick taller and more dramatic
  • The supersaturated solution can be stored refrigerated for a day or two if the container is fully sealed and undisturbed