Kids' Chemistry: Phase Changes

I think the 2 most essential things for kids to understand about phase changes is that atoms and molecules are attracted to ("stick to") each other, and that phase changes require energy.

The reason that everything isn't solid is that energy (which we perceive as heat) makes the atoms and molecules move, and the faster they are moving the harder it is for them to "stick" to each other.

To illustrate this, I had the kids play Crack the Whip.  If you haven't played in a while, the kids hold hands in a line and one kid on the end starts walking, then running, making curves.  When they are moving slowly, it's pretty easy for all the kids to stick together.  As they speed up, it gets harder and harder, and eventually, the kids on the end go flying off.

That's a lot like what happens when water boils!

We talked about the phase changes:
Freezing (liquid to solid)
Melting/thawing (solid to liquid)
Evaporation/boiling (liquid to gas)
Condensation (gas to liquid)
Sublimation (solid to gas - think dry ice)
Deposition (gas to solid - this is how artificial diamonds are made)

Then I had them all be molecules and, as I called out phase changes, they altered their motion to suit the change.  Very fun!

We next worked on altering phase changes.  I had put a pot of water on to boil, and we measured it's temperature: 212 degrees.  The idea that water doesn't get any hotter than that, even if you add more heat, was a bit of a new concept for them, so I stayed with that for a while.

I also explained the Fahrenheit and Celsius systems.  Then I dumped 2 cups of salt into the boiling water and got the temperature up to a whopping 227 degrees!

Never one to be wasteful, I then put the potatoes in to cook for dinner.

Next we altered things in the other direction.  I had two jars with ice and a little water.  I put rock salt into one, and then had the kids feel the difference in temperature between the jars. Given enough time, and a cooler room, the salt water jar would have gotten frosty.

Breaking the bond between sodium and chlorine in the salt takes energy, and the salt gets that energy from heat in the ice.  This doesn't make it warmer, it makes it colder because more energy has been absorbed.(converted from heat into chemical energy).

I used to wonder why the ice would melt if the salt was making it colder.  The answer is that the charged sodium and chlorine ions pull the polar (charged) water molecules away from the ice crystals, back into a liquid state.  Interestingly, 0 degrees Fahrenheit is the temperature at which a saturated salt solution freezes.  I still like Celsius better.

Anyway, to get back to the greater significance of salt lowering the temperature of ice, this is how we make ice cream by chilling the container below the freezing point of cream by harnessing the power of ice and salt! Mmmmm.  Ice creammmm.

Anyway, we also picked up ice cubes with thread.  We had mixed success with this because the room was very warm, but the idea is to lay the thread on the ice cube, then sprinkle salt over it.  The salt melts the ice, but chills it enough to refreeze around the thread.  You need cotton thread for this.

Lastly, we looked at evaporative cooling.  This is a good way to concretely experience the need for energy in order to make a phase change.

I had a bottle of alcohol and a bottle of water at room temperature.  I put some of each on cotton balls, then rubbed them both on kids' arms.

I happened to have a scanning thermometer that records surface temperatures (thanks Mumpy!), but you can feel the difference in any case.  The water feels one temperature as it goes on your arm, then the whole area feels cooler as the water evaporates.  In order to make the jump from liquid to gas, the water has "stolen" heat energy from your arm.  You feel the loss of heat as "coolness." That's why sweating works!  The alcohol does the same thing, but, since it evaporates faster, it removes energy more quickly and feels cooler.