Watch this science video tutorial from Nurd Rage on how to make potassium nitrate from instant cold packs and potassium hydroxide.
Warning: This reaction will produce large quantities of toxic ammonia gas. Perform the reaction outside or in a fume hood, and wear gloves when working with potassium hydroxide.
Some instant cold packs contain ammonium nitrate as their active ingredient and when mixed with potassium hydroxide will produce potassium nitrate and ammonia gas.
Get an instant cold pack that clearly says it contains ammonium nitrate, cut it open, and pour the contents into a container. If its unused there will be a water pouch that you can discard. The rest of the pack is ammonium nitrate. Usually its coated in an anti-caking agent so it'll be off-color.
If you're using an already used instant cold pack then filter out the liquid and let it evaporate until it's dry. The crystals will be ammonium nitrate.
Take 80 grams of ammonium nitrate and dissolve in 70 milliliters of hot water. This will take some time so be patient. It will take even longer in cold water so use hot water when you can.
If it has an insoluble anti-caking agent then you need to filter it off through a coffee filter.
Separately, measure out 56 grams of potassium hydroxide. Potassium hydroxide is sold online to make homemade soaps and for biodiesel.
Add just enough water to completely dissolve the potassium hydroxide.
When both solutions are clear and ready, add the ammonium nitrate solution to the potassium hydroxide solution. This step will produce large amounts of ammonia gas and must be done outside or in a fumehood.
Leave the solution in a very well-ventilated area (outside is best) for all the water and ammonia to evaporate. You can also boil the solution to dryness, just remember it is still producing ammonia.
Brought to you by one of WonderHowTo's favorite scientists NurdRage.