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Floating Rock/Sinking Wood

This demonstration introduces the students to the idea that large objects do not necessarily have a higher density than small objects

Density depends on how much mass is packed into a given volume (i.e., if you had the same volume of two things, the one with more mass would have a higher density). 

Just because something has more mass (or is bigger) does not mean it is has a higher density. A quick way of describing density is to describe an object as heavy or light for its size.

Pumice stone, unlike regular rock, does not sink in water because it has a low density. Pumice stone is igneous rock formed when lava cools quickly above ground (lava froth). You can clearly see where little pockets of air have formed. 

Ironwood is a common name for a large number of species of wood that are very hard. Our “local” species, the American Hop Hornbeam (Ostrya virginiana), is found in central and southern Ontario. This very hard wood is resistant to compression and is valued for making tool handles and fence posts. An ironwood branch is very dense and sinks in water. 


  • Demonstrate how the distribution of particles in a substance determines its density.


  • Per Demo or Group:
    1 large, clear container of water
    1 large piece of pumice stone (can be ordered through Teacher Source or Boreal or purchased at a drugstore)
    1 smaller “regular” rock
    1 small branch of ironwood (from Teacher Source search for ‘wood’)
    1 larger “regular” wood branch

Key Questions

  • How does the ironwood stick compare to the regular wood stick?
  • How does the pumice stone compare to the regular rock?
  • How could you measure the relative densities of unknown objects?

What To Do

  1. Show the students the two sticks and two rocks.
  2. Ask them to predict what will happen when each object is placed in the water. Ask them what they base their predictions on.
  3. Put the objects in the water individually or as pairs. (The regular rock and piece of ironwood will sink; the pumice stone and regular stick will float.)
  4. Let the students handle the objects to feel the weight (mass) of each.


  • Explain that the density of a substance or an object remains regardless of its size. Cut or break off a small piece of wood from the larger piece. Show that they both float, despite the small piece being “light” and the big piece being “heavy.”
  • Try this activity with different brands of soap bars. Predict which brands (if any) will sink and which brands (if any) will float. Why do some kinds of soap float and some kinds sink? (HINT: Dove soap contains air bubbles.)