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Have you ever come across a smooth pond like the one above? It just seems to ask you to "skip a rock on the surface"! You know when the rock stops skipping, it will sink beneath the surface. What makes rocks sink, while leaves or twigs float in water? It's density!



Remember the properties of moving water. Dense objects like large rocks stay at the bottom, while lighter objects like silt and dirt can be carried away by the flow. Density affects the sorting of naturally occurring materials in a mixture.

In this activity you will demonstrate how materials can naturally sort based on their density.



  • A container with a lid
  • Various naturally occurring objects of different densities (sand, gravel, leaves, twigs—do not include any man-made materials!)
  • Water
Safety concerns: icon Be sure to keep all glassware, and chemical object safety rules. Remember not to put a stopper into any flask unless given direct instruction by your teacher. As with all science lab activities, the most important safety rule is to follow all teacher directions.


Using the materials listed above, design and conduct an experiment to demonstrate how materials can naturally sort based on their density.

Need help? Here is an example of a possible procedure you might do.



  1. Draw a picture of the container just after you finish shaking it.
  2. Carefully observe your experiment.
  3. Record your data:
Time to settle out
  1. Draw a picture of the jar after it has finished settling.


  1. How much faster did the first layer settle than the last layer?
  2. Are there objects which move to the top?
    • Describe these objects.
  3. How does this activity relate to the structure of a riverbed?
  4. In a riverbed where would you expect to find the biggest rocks?
    • Explain your answer.
  5. In a river why do you find sand and mud around slow moving water?

Review science safety rules here.

Get the plug-ins: Get Adobe Acrobat Reader , and Get Quicktime Player. (The Quicktime plug-in is needed to play sounds and movies correctly.)

Want to share photos of you or your friends doing this activity? Send it in an e-mail with the following information:

  1. The title of the activity
  2. The URL (Internet address)
  3. Your name.

Remember that no pictures can be used that show student faces or student names on it. 

Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 7th grade science core.

Updated October 24, 2008 by: Glen Westbroek

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