Density tricks with LEGO® bricks

First published in Chem 13 News, February 2003

Authors: D. J. Campbell*, R. A. Bailey, Department of Chemistry, Bradley University, Peoria IL  61625, E-mail: campbell@bradley.edu

LEGO® building bricks are a relatively common household toy.  Therefore,Lego diver in a bottle students who use these bricks as part of educational demonstrations will find some degree of familiarity with these materials.  While measuring the densities of the bricks via water displacement it was noted that many air bubbles can adhere to the inside and outside of these irregularly-shaped plastic pieces.  Whereas the formation of these air bubbles on the bricks has been largely a complication in studying the densities, they do enable the bricks to be used as Cartesian divers.  Cartesian divers have had a long history both as novelty items and as science demonstrations.1-4  These divers usually consist of some object that is more dense than water attached to air bubbles.  The object and its associated air, such as a soy sauce packet with a bubble in it,4 can be thought of as one unit.  This unit is typically enclosed in a flexible container, such as a plastic bottle.  At normal pressures the unit is not sufficiently dense to sink.  When the bottle is squeezed, however, the incompressible water com-presses the air in the bubbles.  As the air bubbles get smaller, the unit becomes more dense and sinks.  D.J.C. has used the Cartesian diver in his classroom to illustrate principles of density, the compressibility of air, and the incompressibility of liquids.

To make a LEGO® Cartesian diver the following materials are needed:

  •  LEGO® person (This process works with other LEGO® brick
     too, but the person-shaped diver certainly looks the best.) 
  • empty plastic soda bottle
  • paper towels
  • tap water

1)   Fill the soda bottle completely with tap water running at a trickle from a faucet to minimize air bubbles.  Cap the bottle and let it sit for a minute or so until all the air bubbles have risen to the top of the bottle.  Banging the tightly closed bottle on a countertop helps to shake the bubbles loose from the walls of the container.

2)   Remove all accessories (including hat, hair, etc.) from a LEGO® person, leaving the head, arms, hands, body, and legs.  Point the arms upward.

3)   Run the LEGO® person under water to wash away some of the air pockets, then place the LEGO® person into the bottle.  Some of the water displaced by the LEGO® person will be pushed out of the top of the bottle and can be mopped up using the paper towel.  Even though the ABS plastic from which LEGO® bricks are made is more dense than water, enough air is trapped in the LEGO® that it floats.  If it does sink right away, retrieve the LEGO®, dry it off, and start the process over again.

4)   Add water to the bottle if necessary to make sure that it is as full as possible.  Carefully screw the cap onto the bottle so as to spill as little water as possible. (For example, do not squeeze the bottle while securing the cap).

5)   Squeeze the bottle.

If the LEGO® person sinks when the bottle is squeezed and then floats up when the bottle is released, then you have made a Cartesian diver that actually looks like a diver (see above).

-  If the LEGO® person stays floating when the bottle is squeezed or you have to squeeze really hard to get the LEGO® to sink, try banging the tightly closed bottle in different orientations on a countertop to shake the bubbles loose from the LEGO®. Then when the bottle is upright remove the cap, make sure the bottle is as filled with water as possible, and re-cap the bottle.  It should be easier to get the LEGO® person to “dive”.

The composition of the LEGO® bricks depends on the type of brick.  Most of the opaque bricks are made from an acrylonitrile-butadiene-styrene (ABS) copolymer.  (Most of the translucent bricks are made from a polycarbonate polymer.)5  Two-peg by two-peg ABS bricks (which have an average volume of approximately 1.13 cm3) have densities around
1.2 g/
cm3, and there appears to be some connection between color and density of the bricks.  We have found that the densities of the bricks, from least to most dense, are, approximately: red @ black < green @ yellow @ blue < white.  Unfortunately, there is enough variability in the density of the bricks that this ordering is not absolute.  This also does not seem to match the ordering of the bricks by mass (green bricks were often the lightest).

These density differences in the bricks do allow them to be separated by aqueous salt solutions; solution techniques have been used to separate plastic samples in the past.6,7  A solution with a concentration of
5.8 g/100 mL of water (roughly 3 teaspoons NaCl/1 cup water) has a density close to that of the bricks.  The solution can be made more dense by adding more salt or less dense by adding water.  Bricks that are more dense than the salt solution will sink, bricks that are less dense will float.  Laboratory-grade sodium chloride should completely dissolve in the water, but salt from a grocery store can contain a certain fraction of insoluble material.  The water should be degassed (i.e., by boiling) before making the solution in order to minimize air bubbles being trapped in or on the bricks.  Air bubbles can be removed from the bricks by using two pipets: one to hold the brick under water and the other to flush the bubbles away from the brick itself.  One way to demonstrate the density differences of different bricks is to sink bricks of various colors in a large open beaker of salt solution.  As the water slowly evaporates, the salt solution becomes more dense and different bricks rise at different times.

Acknowledgements

We would like to thank Bradley University, the National Science Foundation through the Materials Research Science and Engineering Center for Nanostructured Materials and Interfaces (DMR-96325227), the LEGO® Corporation, and the American Chemical Society – Project SEED for generous support.  D.J.C. would also like to thank Kathleen Shanks-Rehder for introducing him to the soy sauce packet variation of the Cartesian diver.

Note

1.   LEGO® is a trademark of the LEGO® Group.  Demonstrations and experiments using LEGO® bricks can be found at http://www.mrsec.wisc.edu/edetc/LEGO/index.html.

References

1.   M. Sarquis and J. Sarquis,  Fun with Chemistry, Vol. 2; Institute for Chemical Education: Madison WI, 1993; pages 121-167.

2.   J.U.S. Thompson and K.A. Goldsby, Journal of Chemical Education, September 1994, page 801.

3.   K.D. Pinkerton, Journal of Chemical Education, February 2001, pages 198-200B.

4.   E. Mueller, Physics Teacher, 1996, Volume 34, page 296.

5.   A. Demers, LEGO Educational Division, Enfield CT.  Personal communication, 2002.

6.   K.E. Kolb and D.K. Kolb, Journal of  Chemical Education, April 1991, page 348.

7.             R. Bruzan and D. Baker, Journal of Chemical Education, May 1993, pages 397-398.