Damn.
Somehow I was able to get through high school and college without ONE chemistry class, and NOW I’m regretting it.
I recently had to ice my knee (Stupid basketball game), and I used an INSTANT COLD PACK, which basically mixes water and ammonium nitrate (AN).
Break the inner seal and WHAM, cold as cold can be.
How does it work? What does simple H2O do to AN to create that wonderful soothing cold for my aching knee?
Thanks.
When you squeeze the cold pack, it combines the water and the ammonium nitrate (NH[sub]4[/sub]NO[sub]3[/sub]). Ammonium nitrate is a salt and it gets dissolved in the water. Unlike mixing regular table salt and water, this is an endothermic solution, so it requires energy from it’s surroundings to dissolve the salt into the water.
Obviously, this energy is in the form of heat. It basically sucks the heat from it’s surroundings to complete this process and that is why the pack feels cold.
Any chemists out there can probably give you a more detailed answer.
Here goes, as I am a chemistry teacher, I should be able to explain this one well.
Dissolving of something involves three seperate but linked processes. It’s important to know some things before I describe them, including the following two axioms:
1)Processes that involve the formation of bonds, of any type, are exothermic: they give off heat energy, or make the surroundings warmer. Bonds are any attractive force between atoms, ions, or molecules.
2)Processes that involve the breaking of bonds, of any type, are always endothermic: The absorb heat energy, or make their surroundings colder.
Also is important to keep in mind the definition of spontaneous: requireing no outside energy, either positive or negative. Thus, we have 4 types of energy-transfer processes:
Spontaneous exothermic: make surroundings warmer
Non-spontaneous exothermic: requires the surroundings be made colder to proceed.
Spontaneous endothermic: makes surroundings colder
Non-spontaneous endothermic: requires that the surroundings be made warmer to proceed.
If we have all of that down, I can begin to describe the process of dissolving:
Step 1: In order for the solid to disolve, individual ions or molecules must be broken away from the solid material. This is an endothermic process.
Step 2: As the ions or molecules enter the solution, they in effect take up space, pushing water molecules away. Since this involves some breaking of water-water molecular bonds, it is also an endothermic process.
Step 3: Water molecules form bonds to the solute molecules, a process known as * solvation *. This is an exothermic process.
Basicly, if steps 1 and 2 involve more energy than step 3, the whole process of dissolving will be endothermic. If step 3 involves more energy than 1 and 2, the whole process of dissolving will be exothermic. This will be dependant entirely on the particulars of the substance being used. Other factors determine if the whole process is spontaneous or not.
For instance, sodium hydroxide or sulfuric acid have VERY high heats of solvation, but reltively low bond energies, meaning that as you dissolve them in water, they are HIGHLY exothermic. Sulfuric acid dissolution is so endothermic as to be able to actually boil the water dissolving it.
In your case, ammonium nitrate (NH4NO3, not AN by anyone who would want to appear remotely educated) is highly soluble, but contains large ions, which means that step 2 above absorbs a LOT of energy, while step 3 (solvation) is relatively small. The net effect is that ammonium nitrate will chill the water dissolving it.
To put it in as simple terms as possible: ammonium nitrate has particular properties that give it a negative heat of solution, meaning that it will absorb heat from its surroundings when dissolved.