Why is the dissolution of potassium nitrate ENDOTHERMIC?
Well, clearly it is a bond-breaking reaction..........
This reaction is favoured by entropy, but disfavoured by enthalpy. How? As usual, entropy wins. I think those portable cold packs (for first aid use) contain a blister of water inside a sample of the potassium salt. When you come to use it, you break the blister (i.e. keeping the pack intact), and the cold pack is placed over the sprain or injury, to reduce the swelling and provide topical relief.