In water, sodium acetate exhibits an uncommon tendency to form "supersaturated" solutions, in which more sodium acetate dissolves than should. These solutions are inherently unstable and, given a suitable surface on which to form, the sodium acetate crystals will readily crystallise back into their solid form. This crystallisation process is exothermic, meaning that it liberates heat. As a consequence, supersaturated solutions of sodium acetate are the primary ingredient of chemical hand warmers, or "hot packs," sold over the counter in chemists. "Activating" the hand warmers induces crystallisation of the sodium acetate. These crystallised solutions are sometimes referred to as "hot ice." This phenomenon also makes an interesting demonstration in which the supersaturated solution is poured from a glass to make a tower.
Purchase sodium acetate, either dehydrated or in its "trihydrate" form. Alternatively, "Popular Science" magazine offers instructions for recovering sodium acetate from the chemical hand warmers sold in chemists.
Add half of the sodium acetate to a saucepan and add just enough water to dissolve it. Too much water will ruin the experiment, so add the water a little at a time. Stir constantly.
Heat the sodium acetate and water solution to almost boiling, then add more sodium acetate 1 tbsp at a time. The solution needs to be saturated, which means that as much sodium acetate needs to dissolve as possible. Therefore, a small amount of undissolved sodium acetate would ideally remain in the pan, confirming that no more will dissolve in the hot water.
Transfer the hot solution to a glass, being certain to NOT transfer any undissolved sodium acetate crystals. Otherwise, the sodium acetate will solidify in the glass. Place the glass in a freezer for about 20 minutes. After cooling, the solution should still be liquid, with a syrupy consistency. This solution is supersaturated.
Place the remaining solid sodium acetate in a bowl or pan; the solid sodium acetate will serve as the seed crystals to trigger crystallisation of the supersaturated solution. Slowly pour the cold supersaturated solution onto the seed crystals. The supersaturated solution should crystallise on contact with solid sodium acetate. Continue to slowly pour the sodium acetate solution to form a "tower."
Sodium acetate is not particularly hazardous, but the use of safety glasses is recommended.
Tips and warnings
- Sodium acetate is not particularly hazardous, but the use of safety glasses is recommended.