The Density Propensity

Today was DENSITY DAY.  No, it wasn't a holiday or really anything too exciting, but I L.O.V.E. teaching physical science ANYTHING, because of the endless possibilities of LABS.  I heart labs.  Kids heart labs, too.  So, I went to the very faithful grocery store and purchased lemons, limes, grapes, eggs, and hard boiled eggs w/out shells. (Did you know you can buy them in bags already de-shelled?  How awesome is that?!  Lazy, yes, but AWESOME.)  I had some pencil top erasers, so I used those, too.  I asked the kids to make a chart of "Float/Sink," and I told the items I had.  I wrote them down, and the kids placed them in their predicted columns of which items [listed above] that they thought would sink or float.  We discussed why they think items float or sink, and there was a pretty big consensus about light items floating and heavier items sinking.  This is when we began our test.  I started out with this container under the document camera:

This container was TOO shallow.  We couldn't tell if these items were floating or sinking.  Fail.  So, we moved to these:

Buckets are nice.  I have lots of them, so we had four large groups.  Students could at least record data according to which items sank or floated.  They were very surprised that the lemon was the only item to float.

I led students to the idea of density.  The cool thing is that we have been learning about atoms and elements, so the students understand the atomic mass in the Periodic Table.  They could look up the atomic mass of Hydrogen and Oxygen, the building blocks of water.

I then asked them to figure out a way to change the density of the water to get more of the items to float.  After some pretty creative ideas, I asked if anyone had heard of the Dead Sea.  At least one person in each class had and could tell something about the salt content.  After mapping the Dead Sea on our world map (where we map everything), the students began adding salt to their buckets of water to try to change the density of the water.

After about a cup and a half of salt, the hard boiled egg floated, followed by the raw egg.  Even with an entire container (about 3 cups) of salt, the eraser and grape did not float.

This surprised students and proved their theory about weight being the determining factor to be wrong.

Students found salt (sodium) on the periodic table, and realized the atomic mass is larger than that of water.  This explained why more items floated in the salt water, and why salt water is MORE DENSE than water.

Ah, science.