Kids' Chemistry: Heat

  We've been discussing heat from the very beginning, so the kids knew that heat was energy, and that heat energy was expressed in the movement of molecules.  You've got to love it when the kindergarteners can tell you that molecules move faster when they are hotter, and slower when they are colder.  They get it!

Anyway, they also remembered that heat caused an increase in reaction rates: it sped up the dissolution of the bouillon cubes, and it sped up diffusion.

 I also wanted them to see that heat affects density in liquids and gases.  I used 2 wide mouthed quart jars, and two little (8oz) soda bottles.

I put ice water in one little bottle and tinted it dark blue, then sealed it with foil and a rubber band.  I filled the quart jar with hot (tap water, not boiling hot) water. Then I pierced the foil with a pencil tip and dropped a few drops of the cold blue water into the hot clear water.

The cold water fell quickly.

This is the basis of certain ocean currents.  In real life, water density based on temperature moves millions, if not billions of tons of water!

Next I filled the other bottle with hot water, dyed red, and sealed it with foil and a rubber band.

I placed the sealed bottle into the wide mouthed jar, then filled the rest of the jar with cold water (strained ice water).

Next I pierced the foil on the hot water with a pencil tip.

The red water rose to the top, looking very much like a volcano!

 We moved on to gases.  Before class, I had placed an empty, uncapped soda bottle in the freezer.  Now I took it out and capped it with a quarter.

Theoretically, as the air in the bottle warmed and expanded, the coin would clatter and move.

Actually, the bottle got knocked over (twice!) and was filled with room temperature air.  So, your mileage may vary! 

Since the bottle was full of warm air, I reversed the experiment by capping the bottle (with, you know, the bottle cap) and stuck that in the freezer.  By the end of the class it was cold enough to shrivel the bottle.  The kids asked what would happen if we left it in all week, so we left it there, plus added a bottle of water.

Next up, we looked at how light was absorbed and converted into heat.  I had a (florescent) light bulb shining on black and white paper.  Theoretically, I would have had a thermometer under each sheet of paper, but a surface reading thermometer was more fun if you have one!

The black paper was two degrees warmer than the white.

 Lastly, we looked at heat absorbed and released in chemical reactions.

I stuffed a jar with fine grade steel wool, then poured in some vinegar.  I sealed it with a double layer of foil, then put a cooking thermometer through the foil into the center of the steel wool.

Over the course of 10 minutes, the temperature went up about 2 degrees- rust really is burning in slow motion!

 Then I dissolved half a cup of non chlorine bleach (Oxyclean) in water.  I had everyone touch the jar before we started so that they could feel that the water was room temperature.

Naturally, everyone wanted to stir!

I didn't use a thermometer on this one, but the kids all could feel the jar growing warmer.

For my piece de resistance, I did "hot ice."  This is sodium acetate, the stuff used in hand warmers.  It's reasonably non toxic, and available on Amazon. 

You need 160 grams of sodium acetate (which is the amount in the beaker) dissolved in only 30ml of water (which is the amount in the graduated cylinder).  It looks like a ridiculous proportion of solid to liquid - and it is!

But this is a case where a relatively low saturation point is very affected by temperature.

 If you heat the mixture in a boiling water bath, the entire amount will go into solution, and it will stay in solution as mixture cools.  This is called a super saturated solution.

And it only takes one tiny sodium acetate crystal (or even a hard tap on the side of the jar) to cause the sodium acetate to fall out of solution.

It instantly blooms into beautiful crystals and releases all that stored up heat you used to coax it into solution.

The best part is that you can put those crystals  (now a solid block with no visible water) back into solution by adding more heat, so your sodium acetate is completely reusable!