coolscience
Boiling COLD?? Freezing Hot?? CoolScience can really heat stuff up! Here’s what probably happened today.
We played with stuff. Liquid stuff, solid stuff, even stuff in the air. All stuff is made out of really, really, really tiny bits called molecules. Those molecules usually stick together like puzzle pieces, to make up the stuff around us. Even if something looks big and solid, it’s really just lots of molecules stuck together.
You are made up of few billion, billion, billion of them!
Molecules are always shaking around. When things feel cold, it’s because their molecules are shaking slowly. When we say stuff gets “hotter” it means its molecules start moving faster. And faster! And faster! Remember when we held hands and started moving around? It got hard to hold hands as we moved faster. If a molecule shakes hard enough, it can break away from the others. This is how things melt or evaporate or boil.
Some molecules stick to each other very tightly, like water. Some don’t. Water needs to take in a lot of heat to evaporate or boil, but nitrogen molecules break away easily and fly through the air by themselves, rarely sticking together. In fact, to get nitrogen to stick together in the air, the molecules would have to be so slow that it would feel -320°F (-196°C) to a thermometer. It would probably freeze the thermometer!
Using a fancy insulating thermos called a Dewar, we brought cold, slow nitrogen molecules stuck together as a liquid. They were so cold that they froze a banana hard enough to hammer a nail into a board! But those nitrogen molecules were so easy to break apart that if any spilled on the carpet, they instantly boiled and flew away into the air, leaving a dry (but cold) carpet behind. 
This is how we cooled down our ice cream. We poured liquid nitrogen right into the ice cream mix. The nitrogen molecules warmed up and started bouncing around, taking heat from the milk and cream. The nitrogen flew away into the air, taking the heat with it.
Wait a second! That sounds familiar: liquid disappearing into the air, taking heat away.
That sounds like…sweating! We sweat to cool off. As we dry off, water takes our heat away into the air. It’s the same thing! The ice cream froze by “sweating” nitrogen!
There were a lot of other cool things you can do by speeding up or slowing down molecules. We put some 
nitrogen in a bottle and saw them go bouncing around in a balloon. The molecules fly around and bump into the walls of the balloon, 
out the balloon. Those molecules sure spread out a lot! A few drops of liquid could fill a huge balloon, and even make it pop!
In the other direction, we cooled down balloons and did a “magic trick” of packing tons of balloons into a tiny bucket. What a great way to pack for vacation! The molecules slowed down, and the balloons shrank, with all the air still inside! Do you remember what happened when we blew on them with our warm breath? The molecules sped up, started bumping into the rubber of the balloon again, and the balloon expanded to its original size!
Try putting your own balloon in the freezer, and see what happens.
Slow down your own ice cream molecules, using this recipe:
½ cup half-and-half |
Put these into a 1-gallon zip-lock bag: Put the little bag in the big bag, and start squishing it around, until the ice cream in the little bag is frozen. Suggestion: wear winter gloves so only the ice cream freezes. |
Put these ingredients into a 1-quart zip-lock bag, 