This fun experiment explores density by discussing molecules, and determining whether objects sink or float.

Before you build your density dunk tank, take a few minutes to review matter and molecules (this is in the video too!). Using tape, make two equal sizes boxes on the floor. Choose an object you have quite a few of in the same shape and size (bouncy balls, dolls, etc.). These will represent molecules. Demonstrate that when you compare two objects of the same shape and size (the taped boxes), the one that has more molecules also has a higher density!

Want to duplicate the density dunk tank at home or in your classroom? Watch the video for an overview, gather the materials listed at the right, and follow the instructions below!

Required Materials

  • large bowl or container
  • water
  • 2 sheets paper
  • pen, pencil, or other writing tools
  • small, similarly sized objects to test

Step-By-Step Instructions

Step 1

Fill a bowl 3/4 of the way to the top with water.

Step 2

Use your paper and writing utensil to make two signs – one with the word “sink” the other with “float“.

Step 3

Review that an object will sink if it has more molecules in its space than the water does. An object that sinks has a higher density than water. Conversely, an object will float if it has less molecules in its space than the water does; an object that floats has a lower density than water.

Step 4

Go through your items and make hypotheses about whether they will sink or float. Place them on the relevant signs according to your hypotheses.

Step 5

Once you’ve made an hypothesis about each of your items, drop each one into the bowl to test it. Was your hypothesis correct? Repeat the process with each object and do some comparisons once you’re done. Which objects surprised you?