Always wear safety goggles and gloves when handling acids and bases like vinegar and baking soda.
The reaction of vinegar with baking soda is as follows: NaHCO3 + CH3COOH —> CH3COONa + CO2 + H2O
You will only have solid baking soda if you added too much baking soda. The excess baking soda will remain in a solid (but wet) form.
If the crystals do not form, the solution may be resting in a supersaturated state. This means that there is excess sodium acetate dissolved in the water for its current temperature. Drop a small piece of metal (even aluminum foil should work) into the solution to start the crystallization. If you are building hot ice sculptures, you will want to pour the solution a little at a time into your mold or design. This should catalyze the sodium acetate to precipitate out of solution, and form a solid sculpture.
If you plan to make a hand warmer, put the crystals in an airtight plastic bag. You can melt the crystals by dropping the bag into boiling water. Leave it in liquid form until you need a hand warmer, then drop a crystal or piece of metal in to catalyze the change back to a warm solid.
A wide/long, shallow dish, like a glass casserole dish, works best. The water will take much longer to evaporate from a deep dish.
