Materials
- 4 cups white vinegar (5% acetic acid)
- 4 tablespoons baking soda (sodium bicarbonate)
- Medium-sized pot
- Stirring spoon
- Heat-safe glass jar or beaker
- Small bowl
- Refrigerator
- Aluminum foil
- Plate or shallow dish
Instructions
- Pour the vinegar into a medium-sized pot.
- Slowly add the baking soda, one tablespoon at a time, stirring gently. Add it slowly to control foaming and bubbling.
- Once all the baking soda has been added and fizzing stops, place the pot on the stove.
- Heat the mixture on medium heat and let it simmer. Continue heating until the liquid volume is greatly reduced and a thin, crusty film begins forming on the surface and sides of the pot. This can take 30 minutes to a few hours.
- Carefully pour the hot liquid into a clean, heat-safe glass jar and cover it tightly with aluminum foil to prevent evaporation and contamination.
- Place the covered jar in the refrigerator for 30–45 minutes to cool without crystallizing.
- Scrape a small amount of the solid crystals left on the sides of the pot into a small bowl. These are your seed crystals
- To create a crystal tower, place a small amount of seed crystals in a shallow dish.
- Slowly and carefully pour the cooled liquid over the seed crystals.
- Observe as the liquid instantly crystallizes and builds upward into a solid, ice-like tower! The solid feels warm to the touch, even though it looks like ice.
What’s going on?
- When vinegar and baking soda react, they produce sodium acetate, water, and carbon dioxide. After boiling off excess water, the remaining solution becomes highly concentrated sodium acetate.
- When this solution cools without forming a solid, it becomes supercooled and unstable. Adding a seed crystal triggers nucleation, causing the sodium acetate to rapidly crystallize. During crystallization, energy is released as heat, which is why the “ice” feels warm. This process is an example of an exothermic phase change.
Safety
- Adult supervision required. The solution becomes very hot during preparation and crystallization. Use heat-safe containers and allow liquids to cool before handling.
Tips for success
- The solution must be very concentrated. If it does not crystallize, reheat it and reduce it further.
- If the liquid solidifies in the refrigerator, add a small amount of vinegar, reheat, and try again.
- Make sure the glass container is very clean. Dust or scratches can cause early crystallization.
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