Discovering Density

 The sixth graders have been working with density in chemistry class.  We start by learning about mass and volume.  The kids learn how to use a balance scale and a graduated cylinder.  By finding the mass and volume of water and then using the density formula (mass divided by volume), the kids discover that water has a density of 1.0 g/ml.  After they know that information, it is easy to determine if objects will float or sink in water.

I then do four density demonstrations.  The first one involves placing a can of coke and a can of diet coke in a tank of water.  The soda has the same volume, but one floats and one sinks.  (I am not going to reveal which is which, but you can probably figure it out.)  The kids realized that the mass must be different and after a quick check of the scale, we discovered they were correct.

The second demonstration is called "dancing raisins."  If you place raisins in a graduated cylinder full of lemon lime soda, they will dance up and down.  As bubbles collect on their surface, the raisins rise to the top.  Once on the top, the bubbles pop and the raisins sink to the bottom.  It doesn't take long for the kids to figure out that the density of the raisin is changing as it goes up and down.

For the third demonstration we layered fresh water (yellow) on top of salt water (green).  The kids learned that salt water has more stuff (salt) packed into the water and so it has a higher density.

Our final demonstration is my favorite.  Cold water (blue) and hot water (red) are placed in small glass bottles and then turned on their side inside a tank of water.  The cold water sinks to the bottom as it pours out and the hot water floats to the top.  This one is harder to explain, but the kids did a nice job of coming up with some good reasons why hot water has less density than cold water.

We are currently working on a challenge where the 6th graders must figure out how much sand to place in a film canister so it will remain suspended in a tank of water.