A cold pack is basically a small plastic bag that can become an instant cold compress. A cold pack is an endothermic reaction, meaning that the reactants inside the cold pack act together to absorb heat. A reactant is defined as a substance that takes part in a chemical reaction. In the case of cold packs, there are three reactants that partake in the reaction of the pack becoming cold suddenly.
The first reactant inside the cold pack is hydrogen. The liquid you can feel inside the cold pack is partially made up of hydrogen.
The second reactant is oxygen. This is mixed with the hydrogen to form the liquid chemical substance inside the cold pack--most commonly known as water.
The third reactant is nitrogen. Floating in the water inside the cold pack is a small tube containing a chemical compound, 27 per cent of this compound is made up of nitrogen.
Also inside the tube in the pack is 8 per cent calcium carbonate. This mixes with the nitrogen to create the compound ammonium-nitrate. It is a fertiliser commonly used in gardening and farming. These reactants are found mixed together inside the tube in a cold pack because hydrating it causes an endothermic reaction.
When the cold pack is shook or bent, the tube inside breaks and the reactants all come together to form the reaction that is the cold pack becoming cold. The cold pack can reach about 1.67 degrees C. for around 10 to 15 minutes.