I need to know how to freeze water outside a freezer.
For what application? There are thousands of different chemicals with endothermic reactions that may be suitable for your specific purpose. I can't think of one at the moment that's food-safe. Whatever method, you have to either mix something into the water, or use a container within a container.
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No real application; just curiosity.
But, thanks for the great responses.
If you dissolve nitrates in water, than it will cool down (e.g.: potassium nitrate, ammonium nitrate). This can cool down under 0 Celsius.
Another way is to pour concentrated nitric acid on ice, it will cool down to minus 80 Celsius.
Are you looking for a way to really freeze water? Ammonium nitrate may be the way to go for a chemical reaction.If you don't mind a physical reaction, you have some easier choices. Make a saturated solution of table salt or sugar, put it in the freezer, and turn the temperature way down. It should remain liquid, but become much colder than water ice. You can then dip your container of water into this for instant ice. You can make ice cream this way. If you want an even colder liquid, you can take pure alcohol or a calcium chloride solution and add dry ice.If you just want the illusion, a good physical reaction is the crystallization of a sodium acetate solution, which looks much like water freezing.
When ammonium nitrate is added to water, it drops the temperature. It is used in the cold packs. It may be enough to freeze water.