Hot ice is another name for sodium acetate (CH3COONa or NaOAc). It is the sodium salt of acetic acid, which is the key component of vinegar. Hot ice gets its name from the way it solidifies. A solution of sodium acetate supercooled below its melting point suddenly crystallizes. Heat is released and the crystal resemble ice so… “hot ice.” All you need to make sodium acetate and crystallize it into hot ice is baking soda and vinegar. It’s a great chemistry demonstration because it illustrates chemical reactions, supercooling, crystallization, and exothermic processes. From start to finish, the project takes less than an hour. Once you have the sodium acetate, you can melt and crystallize it over and over again.
You only need two ingredients, plus a pan and stove:
- 1 liter Vinegar (weak acetic acid)
- 4 tablespoons Baking soda (sodium bicarbonate)
The quantities of baking soda and vinegar are not critical so long as all of the baking soda dissolves. If measuring the ingredients isn’t an option, just dissolve baking soda in vinegar until no more dissolves, filter off the liquid using a coffee filter or paper towel to remove any solids, and proceed from there.
Be sure to use plain white (clear) vinegar and not cider, red wine, or some other colored vinegar. You can substitute sodium carbonate (washing soda) or sodium hydroxide (caustic soda or lye) for the baking soda. If you have access to pure sodium acetate (inexpensive online), you can skip the procedure to make it and go directly to the step for re-using it.
- The first step is reacting the baking soda and vinegar. Stir baking soda into vinegar a little at a time. If you add it all at once, you’ll basically get the classic baking soda and vinegar volcano and could overflow your pan! The reaction between baking soda and vinegar produces sodium acetate, water, and carbon dioxide gas:
Na+[HCO3]– + CH3–COOH → CH3–COO– Na+ + H2O + CO2
However, at this point there’s too much water for the sodium acetate to crystallize.
- Next, concentrate the solution by boiling it. It took me about an hour at medium heat to reduce the volume from a liter to about 100-150 milliliters. Don’t use high heat because you may get discoloration (golden or brown). The discoloration doesn’t ruin the sodium acetate, but the hot ice will look a bit like you made it from yellow snow. You’ll know you’ve boiled off enough water when a crystalline skin starts to form on the surface of the liquid.
- Once you see a skin, immediately remove the pan from the heat. Carefully pour the liquid into a clean container and cover the new container with plastic wrap or a lid to prevent further evaporation. You should get crystals in the pan, which you can use as seed crystals for activities, but the liquid in the new container should not contain any crystals. If you do have crystals, stir in a very small volume of water or vinegar to dissolve the crystals. If the entire solution crystallizes, add more water and go back to the stove to boil it down again.
- Place the covered container of sodium acetate solution in the refrigerator to chill it. It’s also fine to let the solution cool to room temperature on its own, but this takes longer. Either way, reducing the temperature produces a supercooled liquid. That is, the sodium acetate remains liquid below its freezing point.
Hot Ice Activities
Solidification of sodium acetate is the basis for one type of hot pack, but it’s also great for crystallization demonstrations. Three popular activities are the “sea urchin,” “flower,” and “tower.”
- Sea Urchin: Pour the cooled liquid into a clear container. Use a toothpick or bamboo skewer to scrape a few sodium acetate crystals from the pan used to make the solution. Dip the toothpick into the liquid so the tip with crystals are in the middle of the container. Needle-like crystals immediately grow out from the center. Also, crystallization releases heat as chemical bonds form to make the solid. The final structure resembles a spiny sea urchin.
- Flower: Pour the cooled sodium acetate liquid into a flat dish (preferably a dark-colored one). Scrape one or more crystals from the pan and drop them onto the liquid. The crystals act as seeds. The hot ice crystals spread out radially and form structures that resemble flowers.
- Tower: Place a few crystals onto a surface. Slowly pour the liquid onto the crystals. The hot ice solidifies as you pour the liquid, forming a tower (or whatever shape you can manage).
Re-Using Hot Ice
Save the solid sodium acetate so you can use it again without going through the whole baking soda-and-vinegar process. Simply dissolve the hot ice in water and boil off the smaller amount of excess water.
Sodium acetate is a safe, non-toxic chemical, so it’s perfect for chemistry demonstrations. It is used as a food additive to enhance flavor and is a key ingredient in some chemical hot packs. The heat released by hot ice crystallization of a refrigerated solution doesn’t present a burn hazard. However, making hot ice from baking soda and vinegar does involve boiling liquid on a stove, so adult supervision is required. If you use sodium hydroxide in place of baking soda, heed the cautions on the product label.
- ChemEd Xchange (2019). “Crystallization of Supersaturated Sodium Acetate – Demonstration.”
- Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. ISBN 978-0-19-850346-0.
- Seidell, Atherton; Linke, William F. (1952). Solubilities of Inorganic and Organic Compounds. Van Nostrand.