Density can be a tricky topic to teach. Students usually understand the concept of mass. They can feel the difference in something that is more massive (or heavy) than something else. Volume can be shown easily — which one is bigger? What takes up more space? When you put mass and volume together, sometimes students get mixed up. Does heavy always mean dense? Does little mean it will be less dense?

I have a fun activity that addresses some of these possible misconceptions. It uses simple materials and the results turn out beautifully (if students do the lab correctly!).

The set up:

This activity can be used before introducing density as an inquiry activity, or it can be used as a culminating activity. Since my sixth grade students have worked with density here and there, I incorporate the lab after our official density lesson. In the days before, we work on understanding what makes something dense, students practice calculating density and we may even play a few rounds of “Sink or Float.” (This is SO easy for any grade level and students love it! You can either do it as a demonstration, or have small groups do it. Just get a beaker filled with water and random items from the classroom and students must predict whether it will sink or float when dropped in the water!)

Density Rainbow

In the Density Rainbow activity, students find the density of five different liquids and pour them one at a time into a 100mL graduated cylinder. If the students are careful with their pouring, the result is a beautifully layered, colorful cylinder!

Before measuring a liquid, students predict where in the layer it will go. On the top? Bottom? Somewhere in the middle? Because I want to include the math, I make students find the mass and volume (about 15mL) of each liquid, and then calculate the density. Most students see the connection between the numbers they are getting and where the liquid will end up. The higher the density, the lower it will go in the cylinder.

Make sure you tell the students to pour each new liquid carefully! Tilt each cylinder and pour the liquid down the side of the 100mL cylinder SLOWLY! This is especially important with the rubbing alcohol. If students splash, the colors will get mixed and the final result won’t be as pretty.

Here are the liquids I use:

The liquids ready to go!
  • A) Water with green food coloring
  • B) Vegetable oil (no food coloring necessary – it won’t mix well anyway!)
  • C) Blue dish soap (you can use any color of dish soap you would like, but I like the blue best!
  • D) Corn syrup with purple food coloring (this gets very thick and sticky!
  • E) Rubbing alcohol with red food coloring.
I label all the cups so students know what liquid to use when.

Condiment cups are also extremely helpful in this activity. I can prep this lab the day before, pop the lids on all the liquids and not worry about spilling or evaporating. You can find the cups most grocery store in the paper goods aisle. If you have a Gordon Foods nearby, they have huge packs of them too!

The final product!

If you would like to use the handout that I do, here it is: Densityrainbow.doc

Notes and extensions:

Because I have students find the mass, they must first find the mass of the empty graduated cylinder they use to measure with. I am always double checking that they are subtracting the mass of the empty cylinder from the mass of the liquid plus the cylinder. If they don’t do this, their calculated density will be off!

Another extension I do is have the students change the volume and predict if the layers will still be the same. If we used 30 mL of vegetable oil and only 10 mL of water, what will happen? We do this as a group and it leads to the discussion of why adding MORE liquid does not change the density. A few of my quicker students pick up on the fact that when you increase volume, you are also increasing mass, so the density will be the same!

Have fun adding some color to your classroom! If you have any other ways you like to teach density, share them! I love introducing new activities to my classroom repertoire!

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