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Experiment: Salt Water and Buoyancy

This experiment serves to illustrate some of the properties of salty water.  As you do this experiment, be sure to record it in your notebook.  Use the scientific method in a writeup of the experiment.

A characteristic of salty water is that in it things float better than they do in fresh water.  In a fresh water lake you would float a little lower than you would in the ocean.  The more salty the water the higher you would float.  You would float higher in the very salty Great Salt Lake than you would in the less salty ocean.

You can demonstrate this in your kitchen.

Materials:

  • Salt
  • Distilled water
  • 2 clear glass containers
  • a wooden block

Procedure:

  1. Place equal amounts of distilled water into the glass containers.
  2. Into one of the glass containers, place four tablespoons of salt.  Mark the container so that you know which one has the salt.
  3. Mark your wooden block with a fine tipped indelible marker so that it is marked vertically into half centimeters.
  4. Place the block into the distilled water carefully so that the block is upright in the water.  You should be able to read the markings on the side of the block to determine the depth at which the block floats.
  5. Repeat using the salty water.
  6. Compare the results.  In which type of water did the block float the highest: salty or distilled?

I tried this with a marshmallow.  The marshmallow in the salty water floated slightly higher than the marshmallow in the distilled water.  But since I had not marked the marshmallow first, the results were hard to see and the marshmallow started to dissolve.

This experiment works well with fresh eggs.  When placed in fresh water, an egg sinks to the bottom of the glass.  When placed in a glass of water with 1 tablespoon of salt dissolved in it, the fresh egg will float.

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Activity: Studying Buoyancy

You will Need:
  • bottles with caps (same size) 
  • large container of water (a bathtub will do)
1 Fill one bottle with water and cap it.  Put the cap on the other bottle. 
    a) Which one weighs more?
     b) What is the second bottle filled with?
 
2 Place both bottles in the water. 
    a) What happens?

3 Let some of the water out of the first bottle.  Put it back in the water.
    a) Now what happens?  Try it with various levels of water.

4 Go through your house and find various objects.  Ask yourself if they will float or sink.  Then see if they do. 
    a) What did you notice about the things that floated? 
    b) Did some things that are heavy float while some things lighter than them sank? 
    c) Did it depend on the shape of the object?