* .
Introduction
Endothermic reactions are accompanied by the absorption of heat. The dissolving of ammonium nitrate in water is an example of an endothermic reaction. The solution resulting from this mixture is colder than either the ammonium nitrate or the water. This is the simple explanation of what happens in an instant ice pack. The more detailed information will be discussed in the following paragraphs.
Cold Packs
Most cold packs come with a fabric cover made to absorb condensation and to protect the skin from contacting the surface of the cold pack. Cold packs are used for injury or muscle relief. The cold pack contains two bags one containing water and the other containing ammonium chloride. Once the ammonium chloride is
Step 1: NH4NO3 + Heat Þ NH4 + + NO3 -
Step 2: NH4 + + NO3 - + H2O Þ NH4 + (H2O)x + NO3 - (H2O)x + Heat

The first step requires six kilocalories per mole of ammonium nitrate to occur and since this absorbs the heat from the water the surrounding liquid is significantly colder.
Entropy
Heat is usually produced in a stored bond relation so the reaction that occurs in cold packs does not seem to be possible. Since this is an endothermic reaction the chemical bonds that form actually have more stored energy than what they had before the reaction. This does not seem to follow the natural order of things. When water is dissolving the ammonium nitrate it spontaneously gets cold. Why does this occur? According to chemists this occurs due to a process called entropy.
The measurement of disorder in a system is entropy. In chemical reactions the molecules and ions change rapidly from orderly arrangements to disorderly ones. The disorder for some reason is escalated by nature itself. When this occurs in the majority of reactions to increase entropy, heat must be released or an exothermic reaction occurs. But when this disorder occurs at great levels, thus increasing entropy greatly, the heat flow is reversed, an endothermic reaction occurs. This only occurs when the reaction allow the entropy to increase greatly.
Let us examine the cold packs again, the materials are orderly arranged. The pure