How does sodium nitrate cool water?

Reactions can be endothermic, where the reactants takes heat energy from its environment to complete the reaction, or exothermic where the reactants give off heat energy to its environment to complete the reaction. The reaction between Sodium Nitrate and Water is endothermic, so the Sodium Nitrate takes heat energy from the water to complete the reaction, thus cooling it.

Hope this helps,

WarLord :D