Exploring Density through Mysterious "Liquid Layers"

Towards the tail end of the Burlington fourth graders’ exploration of matter I had the opportunity to join Mrs. Finn’s class for an investigation in density. The abstract concept of molecular matter is understandably difficult for such young scientists to wrap their head around. With this in mind we set our sites towards helping students understand:

  • Different types of matter have different densities.
  • Matter that is less dense than the matter around it will rise while denser matter will sink.


I started with a whole-class observation of the mysterious “liquid layers.” The different layers were not identified but it was clear that they were present in the graduated cylinder as each layer was a different color.

“Liquid Layers” initial setup, with pan balance and beakers of honey and rubbing alcohol.

Small beakers of the red fluid (rubbing alcohol and dye) and the gold fluid (honey) were presented to the students who observed that there was an equal volume of the two in each beaker. From here students were asked to predict: “What will happen when I place both these beakers on opposite ends of the balance?”

Most students correctly predicted the honey would outweigh the alcohol, visually demonstrating that the mass of materials could be different even when the same volume of the materials was being measured. From here we discussed the term density and how the more matter a material or object contains in a space, the denser it is. Students played this concept out visually by asking more and more students to fill a small space on the carpet they were seated on.


From here the real fun began. Students returned to their seats where they were presented with a simple worksheet outlining the three main steps of our main investigation. The first was to draw a scientific diagram of the liquid layers. The different ingredients used were now revealed to the students (honey, karo syrup, dish soap, alcohol and air!) who labeled their own drawings as I modeled my own on the whiteboard.

When diagrams were completed, four objects were introduced to the class: A penny, wooden sphere, small plastic bead, and larger plastic cap. Students were asked to make predictions about what would happen when each of the materials were dropped into the liquid layers. Groups of three to four students each argued their points with one another, encouraged to use what they already knew about these objects and how they floated or sank in water. When deliberation was done, students were given the opportunity to record their own predictions on their diagram by adding arrows to where they thought the objects would ultimately settle.

Student diagram of the liquid layers. Time to predict!

Once predictions were made it was time to put them to the test! The plastic objects were tested last as they presented the greatest opportunity for disagreement between students. They also offered an educational opportunity to demonstrate how, despite the cap being larger and heavier than the small bead, both objects settled at the same place in the liquid layers.

Discussion between students and teachers was spirited, particularly over why objects floated or sank. Students needed to be encouraged to try to use words like dense and density over words like weight and “strong” but almost all students remained engaged in the activity throughout and were able to write a reflection of what they learned at the conclusion of the lesson. Some chose to simply rank the density of the objects while others (particularly those who seemed to finish up early) were asked to try to describe why some materials were more dense than others.

Final results: The biggest surprises were the two plastic objects!

As I cleaned up the lesson and the Mrs. Finn lined her students up for lunch kids were still talking about what they saw, sharing with others about how they might repeat the activity at home. “10 stars!” Mrs. Finn exclaimed as she escorted them out the door. I was left to assume the scale was a 10, leaving me with the impression this is a great lesson for others to follow!

About MrMusselman

@BurlMAschools Science Specialist and @CambridgeCollg Science Methods instructor. @NSTA Professional Development facilitator and author of "Think Like a Scientist: Investigating Weather and Climate" NSTA Kids ebook.
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