How to Teach Density (Solids, Liquids and Gases)

Last time I told you about how I introduce my fourth graders to the states of matter.  Today I'll share how to teach density.  Part of our states of matter unit includes an understanding of density.  We tested out a variety of objects.  Although kids as young as preschool can perform "sink or float" tests, fourth graders still have some misconceptions!  I wrote on the board:

Large             vs               Small
Protractor                         Eraser
Scissors                          Foam dice

How to Teach Density of Solid Objects:  Sink or Float?

Many students thought the larger items would sink, while the smaller items would float.  They were only half right!  Our tests showed that the eraser sank, although the plastic protractor floated. 

As a result, we determined that size and weight do NOT relate to density.  Density is about how tightly packed molecules are.  Tightly packed molecules sink, while loosely packed molecules float. 

A real world example I gave was the ship they boarded on a field trip earlier in the year.  Obviously that floats!  And it's bigger than any item they tested today. 

To bring the point home, I have them test materials of their choosing.  They record their predictions and the results.  

Another misconception some fourth graders have about density is that all gases float and are equally dense.  Many don't believe you when you tell them gas takes up space.  And the idea that some gases are lighter than others doesn't stick.

How to Teach Density of Gases

The best tool to help kids understand about varying density of gases is balloons!

"Oh yeahhhh!  Helium!"  Many kids know that if you blow (carbon dioxide) into a balloon, it won't float.  You need to fill it with a helium tank.  What they may not understand is how helium is stored and measured.  Otherwise how can you tell if you're buying the right amount for all your balloons?
How to Teach DensityI explain that when you rent a tank of helium, they charge for tanks of gas by volume.  Volume is similar to liquid capacity.  This chart shows how big the tank must be, depending on how many balloons you want to fill.  

The size of the balloons matters too!  The larger the balloon, the more helium you need to fill it.  If you have sixty 14 inch balloons, you need a larger tank than if you have sixty 9 inch balloons.  

With our Student of the Month assembly this month, we were able to do double duty with the decorations/science demonstration.  I found this helium tank here (affiliate link).  The kids got to prove how many balloons could be filled with our little tank, and the cafeteria got a little more festive in the process!

How to Teach Density of Liquids 

The quickest way to teach density of liquids is attempting to mix oil and water.  After making predictions, students partnered up.  One poured water into a cup of oil, and the other poured oil into a cup of water.  Of course the oil, being less dense, rose to the top. can preview this fun states of matter fun unit to the complete packet.  Along with the activities you've seen on these blog posts, I've included several other printouts for you:

* Density and Mass lesson with activity sheet
* Mixture and Solution lab sheet
* Water Molecule writing prompts (with vocabulary) expository and narrative writing prompts
* Matter Test (multiple choice, open response, and answer key)


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