OUCH! Owwwwwww!  I just hit my knee when I was reaching for a beaker on that shelf.  It really hurts.  What should we do?  We are stuck in this lab.  You are too young to drive and too small to carry me.  Unless you can lift 175 pounds?  No?  I guess we will just have to use the things we have around us.  We need something that will turn cold really fast.

Do you know what would be great?  If we had some ice.  When you get hurt, or when I get hurt, the hurt body part will swell up like a balloon.  Cold will help this swelling go down.  That would make it easier for me to walk.  You know what would be even better?  An instant cold pack is a pouch or small bag that, when squeezed, breaks a layer inside and becomes very cold very fast.  Most of the time, these packs are used to treat sports injuries . . . or science injuries, like when I fall off the ladder.  We do not have packs here, though, so I will have to teach you how to make one.

Instant, now that's fast!

Do not worry.  This will not be hard.  You will need two things.  The first is water.  Easy.  Pour some into that glass there.  The second thing is a little harder.  Ammonium nitrate is a chemical that looks like little white crystals and can be used in fertilizer or . . . bombs.  It's okay, it will not blow up the water, it's just going to change it.  I would never have you do something that would hurt you.  Then we would both be hurt!  Go ahead and pour some of those into that other glass.

Ammonium nitrate; just add water.

Now let's find something to put them in.  A plastic bag will work great.  Just make sure it does not have any holes.  I will hold the bag open while you pour the water and the ammonium nitrate inside.  Now, before you do that, let me tell you what's going to happen.  When a solid dissolves, it becomes part of a liquid so that you can no longer see the solid.  You have seen this happen when chocolate mix melts in warm milk to make hot chocolate.  When you pour these two together into the bag I am holding open, they will become one and something great will happen.

When these two things are mixed together, they will turn very cold very fast.  You can wear a pair of gloves if you want.  Ready?  Pour!  Some reactions let out energy, but others will take in energy from the world around them.  An endothermic reaction will take in heat energy from around it, turning so cold that it changes the water in your bag into ice.  It turns so cold, in fact, that it will stay this way for 15-20 minutes.  Watch as the two mix inside the bag and turn into ice!

That feels so much better!  Even though we did not have an instant cold pack, we could still dissolve ammonium nitrate in some water to make something that is endothermic, that takes away heat from things around it.  This gives me what I need to bring the swelling down.  In just a few minutes, I should be able to walk well enough to drive us.  No, we do not need to go to the doctor.  I am going to buy one of those robot arms to grab things off the shelf for me.


How Stuff Works.  "Cold Packs"  Discovery, 2011.  <http://home.howstuffworks.com/refrigerator7.htm>