The density experiment – part I

We took common household objects to see if they would sink or float


  • Various items found around the house

The items we chose were:

  • polished stone
  • a cork (or rather 3 corks glued together by my eldest daughter for another project)
  • a sea shell
  • a really large button
  • a pumice stone
  • popped popcorn
  • marshmallows
  • other items include: keys, coins, feather, cheerios, screws etc.
  • a clear plastic shoe box
  • blue food coloring


Step 1: Fill container 1/3 to 1/2 full of water and add a drop or two of blue food coloring (helps to see the objects better).

Step 2: Predict whether objects would sink or float and record on data sheet.

Step 3: Place objects one at a time in the water and record observations.


The girls took turns guessing at which items would float and why. Then we observed the results.

Of course they understood why the rock sank and the popcorn and marshmallows float. The rock was more dense than water, so it sank and the popcorn and marshmallows were less dense than water so they floated.

The pumice stone was a little more tricky, as it is a stone that floats.
Why does pumice stone float?

Even though it’s a stone, it’s a type of lava rock that is full of holes and their are pockets of air in those holes, making it less dense than water, so the pumice stone floats.
Why does the button float when it’s flat, but sink when it’s on it’s edge?

According to this site, the Mississippi River Landmarks and Legends website,

Every object takes up space or volume. All objects in or on water push some water aside, because they need room to take up space. This is called displacement. Buoyancy is the force of water pushing up on something. Sometimes it’s enough to make things float.

If you displace more water, the resultant upward force will be greater, and possibly great enough to make the object float.

There are two forces that are at work here: one that is pushing down on the object (gravity) and one that is pushing upward from the water (bouyancy). When a small, dense object is placed in water, there is not enough upward force to make it float. However, you can make certain things float by changing the shape of the object.

So how do you make other things float by changing it’s shape?
You can sometimes make something float by changing the shape of it so that more volume is in contact with the water, and it’s weight is distributed over a larger area of water, so less upward force is needed at any given point on the object. A button laying flat on the water distributes it’s weight over more area and it floats, yet laying on it’s edge, its weight is concentrated in a smaller area – hence, it sinks. The weight of the button remains the same, but more of the weight is distributed over a larger volume of water when it is laying flat.To further explain this concept, you can try this experiment (taken from Ask a Scientist)

Experiment 2:


  • Aluminum foil


Step 1:  lay a flat sheet of aluminum foil in the water and observe

Step  2: Scrunch the sheet of aluminum foil into a ball and observe.

What happened?

The flat foil floats and the foil ball will sink.

Experiment 3:


  • Toy boat
  • coins


Step 1: Place the toy boat into water.
Step 2: Observe what happens as you add coins to the boat.

What happens to the boat without coins? It floats

What happens to the boat with coins added? It sinks.

Here is a free printable data sheet to go along with this experiment

This entry was posted in 5 minute science bites, Density, Physics. Bookmark the permalink.

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