Chemistry: Forces and Attraction

This class focused on molecular interactions, but we also talked about gradients and chromatography.  With kindergarteners?  Sure, they loved it! 

First up, it was the first warm day of early spring, and the kids came in with a ton of energy!  I had them hold their arms down by their sides and run around in the relatively small space of the kitchen.  Amazingly, no one bumped into anyone.

Then I had them repeat that trick with their arms held away from them.  Of course, they were bumping and tangling all over!

I explained that some molecules were smooth, and some had "arms" sticking out.  The smooth ones tended to glide past each other without interacting, and the more branched molecules tended to get stuck on each other.  I used the example of alcohol (which is "thinner" than water) and oil (which seems "thicker," but is actually less dense than water).

We also talked about the speed the molecules are going (with more heat, the molecules are moving faster and are more likely to interact). Lastly, I told them that some molecules have a small electric charge, positive on one end and negative on the other (polar molecules).  These charges attract each other, and cause the molecules to stick together a little.

The most common of the polar molecules is water, and the easiest way to observe it is to look at surface tension!  I had intended to show them a cup with water brimming over the walls of the glass, but I missed that some how.


I had a large bowl filled with water and ten toothpicks floating in a group on the surface.  I dipped the tip of another toothpick into dish soap, and the kids took turns touching the water between  the toothpicks with it.  The soap breaks the molecular bonds of the water (that's how soap works, it breaks the bonds of the dirt) and the toothpicks fly apart as the water draws away from the broken bonds.

Then I gave each kid a square of wax paper and dropped drops of colored water on the paper.  They each took a wet toothpick and poked and played with the water droplets.  I think, if I had let them, they could have spent the entire half hour playing with the drops!  It's quite fascinating to see the little spheres connect up and roll around the paper.

Next we set up a chromatography experiment to demonstrate capillary action.  I had cut strips of paper towel marked with a pencil line 2 inches from the end.  I gave each kid 3 strips, and they traced each line with a different colored (water based) marker.   Then the put the ends of the strips into jars with a little bit of water.  The idea is that the water level is below the colored strip.  The water rises by cappilary action, and separates the dyes from the markers (and looks super cool!).

After that, I set out squares of foil, each with a small pool of colored water.  Unlike the wax paper where the water balled up, the water on the foil spread out into a thin layer.  Then I went around and dropped (with an eye dropper) alcohol into the center of the water.  The water drew back and shimmered along the edge of the alcohol!  It's an amazing effect!

To show a little more about the differences and significance of surface tension, I plugged two straws with clay (on one end).  One I filled partly with colored water (you can't see the level if you don't color it), and one with colored alcohol.  Then I turned them both upside down in the sink.  The alcohol ran out immediately, but the water (which has much greater surface tension) stayed put in the straw no matter how long we held it upside down!


 Lastly, I took a tall thin jar and filled it partly with colored water.  Then I tilted the jar and slowly poured in alcohol.  The less dense alcohol forms a layer on top of the water. Then I dropped small drops of cooking oil into the jar.  They formed tiny perfect spheres in the interface between the water and alcohol. 

If you wanted to, you could make an impressive stack of density gradients.  You could use a layer of dark corn syrup, then dish soap, then water (maybe color the water?), then oil, then alcohol.  The trick to getting a smooth gradient is to tilt the jar and pour slowly.

We used to make continuous concentration gradients in the lab by layering different concentrations of sugar solutions and letting them meld over night.  You couldn't see any layers! What good is that?  You use it to sort messy combinations of proteins.

You would layer your proteins on top of the "stack" of gradients and then spin the whole deal in a centrifuge.  The different proteins would "find their level" in the different densities, and you could remove just the one you needed.