[GreenAssailant]

"Cold packs mostly work on the same principle except that dissolution of the solid chemical compound absorb heat causing the tempreature of the water to drop. The reaction is endothermic. NH3NO3 is the compound most often used for cold packs"

Why does the temperature of the water drop when the solid is gaining heat to dissolve? Wouldn't the heat put into the solid to make it dissolve be released into the water when it dissolves, causing the water to heat up?

[LackOfFuel]

Like you've written by yourself, it's an endothermic reaction. And endothermic reactions tend to "absorb" the heat, producing lower temperature of surrounding area.