What makes Instant cold packs freeze or get really cold?

I am working on a project with my Son and those cold packs have a liquid or crystal that activate to make them get really cold or freeze. What we need to know is what ingredients make them do this and what activates them to do this?. This will be make my Sons life alot easier to get his project done, he says he can't find out this info.. thanks

Answer:
When certain substances are dissolved, they absorb heat. This is called and "endothermic" process. Other substances give off heat when dissolved; these are "exothermic" processes. When sodium hydroxide is added to water quickly, the solution gets so hot it gets close to boiling. When ammonium nitrate is dissolved in water, the solution can get very cold to the point that a lot of condensation will form on the vessel very quickly. This would be the basis of the instant cold and hot packs. Note that these are not chemical reactions, just stuff dissolving in other stuff. The 1st answerer is correct about ammonium nitrate being one of the ingredients in an instant cold pack. To find out more about the thermodynamics of ammonium nitrate solutions, see the website below (warning: this is a college level lab description).

I have never used an instant cold pack, but I would think a resonable guess is pressureized CO2. I also did a google search on 'instant cold pack ingredients' it came up with Ammonium nitrate and water.

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What makes Instant cold packs freeze or get really cold?

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