There are a few different ways you can make your own chemical cold pack. You could mix citric acid and sodium bicarbonate or you could mix barium hydroxide with an ammonium salt. If you have baking soda and vinegar, you can prepare your own hot ice or sodium acetate and then use the hot ice to make a cold pack. This method is pretty neat because crystallizing the sodium acetate generates a noticeable amount of heat. Dissolving the hot ice then absorbs the heat, so you can use the same chemical to make a hot pack and then a cold pack. Here's all you need to do:
Hot Ice Cold Pack
- plastic bag with zipper
- hot ice
The hot ice needs to be sodium acetate trihydrate, which is the hydrated hot ice that you get right after you crystallize it. If you only have dry sodium acetate you need to dissolve it in the minimum amount of water and crystallize it.
Now, just place your hot ice in the baggie and add a small volume of water. There you go… an instant cold pack! The reaction won't get super-cold (only about 9-10°C), but it's enough to be noticeable, plus the chemicals are re-useable.