With vinegar and baking soda, you can make ice that warms your hands when you press them against it. Amaze your friends and make your enemies burn with envy when you master the secrets of "hot ice."
Sodium acetate is the substance that forms when positive sodium ions get together with negatively changed acetate molecules, C2H3O2(-), and form a solid. This demonstration works because those ions don't have to form a solid right away. Under the right conditions, they can first drag their feet about it, and then rush to solidify all at once. How do they do this? Supersaturation.
A lot of liquids will dissolve solids like water dissolves salt. Pour more in and more will dissolve until eventually the entire liquid can't take anymore. This is saturation. Under certain conditions - often with the addition of heat - the liquid will take more than its usual share of solid. When those conditions change, though, the solids will re-form and crystallize.
To make sodium acetate all you need to do is pour a lot of baking soda into vinegar. To saturate the thing takes a little more work. Add four tablespoons of baking soda to a liter of vinegar. Add it slowly so the bubbling doesn't cause it to overflow the container. Next, fire up the stove and boil it, stirring constantly. If it smells godawful, you're doing it right. You'll need to boil it down until only about a tenth to a fifteenth of the solution remains.
Make sure there are no crystals in the solution. One crystal will cause the whole thing to crystallize. If you see a few, add a bit of vinegar or water to dissolve them. The solution is now supersaturated. It will snap to its solid state when the temperature goes down enough. Here's the thing about supersaturation - like a precariously balanced tower, it still needs something to tip it before it goes. You don't want to give it a chance to tip for a while, so be careful. Pour it into a small container and cover it so it doesn't evaporate. Put the container carefully in the fridge. Let it get cold. What you have now is both supersaturated and supercooled. It's a liquid that contains way too much stuff to remain liquid. It's also a liquid that stays cold well below its natural temperature for turning into a solid.
Take the liquid out of the fridge. Touch it. It should begin to crystallize the moment you touch it. It'll also give off an amazing amount of heat. What you have is a liquid - in which all the molecules are moving and trembling all the time - turning into a solid - in which the particles are locked in place. That means a lot of energy from motion has to be lost fast. The crystals dump their energy into the surrounding air, and give off a ton of heat. The overall impression is of "hot ice."
(Incidentally, sodium acetate is harmless unless you swallow the whole brick. It's a food additive, so you're probably already eating it if you're snacking while reading this.)