Hot/cold packs are used by athletes to minimize swelling of injuries such as muscle and joint sprains. Chemicals can store energy and release it in the form of heat. Exothermic reactions are chemical reactions that release heat. Endothermic reactions are chemical reactions that absorb heat.
The chemistry behind cold packs:
The liquid inside the cold pack is water. In the water is another plastic bag or tube containing ammonium-nitrate fertiliser. When you hit the cold pack, it breaks the tube so that the water mixes with the fertilizer. This mixture creates an endothermic reaction -- it absorbs heat. The temperature of the solution falls to about 35 F for 10 to 15 minutes.
The chemistry behind reusable heat packs:
The liquid inside the pouch is actually a supersaturated solution of sodium acetate. A supersaturated solution is very unstable and crystallizes very easily. However, the crystals require a 'site of crystallization' in order to kick-start the crystallization process. By flipping the disc, the gratings on it snap together and form a "crystallization site" which causes the solution to crystallize into solid sodium acetate again. Crystallization of sodium acetate is an exothermic reaction, hence the pouch heats up. After all of the sodium acetate has been crystallized, the heat will be lost and the pouch will cool down leaving a clump of solid sodium acetate inside the pouch. This reaction is reversible. To get the crystals to redissolve back into the solution, simply throw the pouch into hot water.
Key words explained:
1) Exothermic reactions - chemical reactions that release heat.
2) Endothermic reactions - chemical reactions that absorb heat.
3) Ammonium-nitrate fertiliser - a chemical compound made up of about 27-percent nitrogen and 8-percent calcium carbonate
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