Density in Relation to Float and Sink

Carolyn C. Roberson-Ellis       Morton Career Academy 
                                401 N. Troy
                                Chicago, Illinois 60624
                                312-265-7392

Objectives:

     (Grades 6-8)
1. To create a density column from household liquid solutions.
2. To calculate the density of an object.
3. To determine why objects will float or sink in relation to density.
4. To predict the density of unknown solutions.
5. To introduce Archimedes' Principle.

Materials:

The following materials are needed for each group of 3-5 students:    
    wood (small rectangular or        baby bottle or olive jar
          square shape)               graduated cylinder
    metal screw or nut                balance scale
    plastic (small rectangular or     ruler (metric)
           square shape)              large plastic container
    corn oil                                or small aquarium
    corn syrup                        paper or plastic cups
    food coloring (for water)         small piece of rubber         
    glycerin                          aluminum foil
                                         
Strategy:
                  
Instructors Preparations and Student Activities:
1. The individual student stations should be set up before students arrive.  Put 
   several drops of food coloring into the aquarium.  Use different colors for 
   each group with matching cups.  (optional) 

2. Each station or group will have the unknown household solutions within the 
   cups.  Students are given the information in regards to water (density = one 
   gram/cm3).  Pour the unknown solutions into the baby bottle.  Students will 
   observe the various columns formed. 

3. One should introduce the concept of density using examples and formula, 
   density = mass/volume.  Review methods of computing volume.  One should 
   include the water displacement method too. 
    
4. Each group should have an object for which to compute the density.  For 
   example, a small piece of wood, plastic, metal screw or nut, and rubber 
   should be used to determine the density. 

5. After computing the density, students are to predict whether or not the 
   objects will sink or float in regards to density.  Water is used as your 
   medium (1 gram/cm3).  Make a graph and place all predictions of the board. 

6. After making predictions, students are to drop the objects into the aquarium 
   to observe whether or not the objects will sink or float.  Remove objects 
   from the aquarium and drop the objects into the baby bottles with the unknown 
   solutions. 

7. Knowing the densities of the given objects, students are to predict the 
   density of the unknown solutions.  Students will make predictions based upon 
   where within the jar the objects floated.  For example, did the object float 
   above or below the water?  Would the unknown solution's density be higher or 
   lower than that of the density of a known object (wood, plastic, metal, 
   rubber). 
   
8. After making predictions, give students the names and densities of the 
   unknown solutions.  Students can compare their predictions.  A graph should 
   be constructed in regards to Mass versus Volume. 

Supplemental Activities:

     To introduce Archimedes' Principle, students will construct a displacement 
container using a two liter pop bottle.  They should cut the top and construct a 
spout by cutting the side of the bottle 2cm wide and 6cm to 7cm long.  Students 
will discover, when objects are put into the water, the water will rise or be
displaced.  (Use heavy objects to make observations, for example a can of pop 
with a known mass.)  Archimedes found that the amount of water displaced is 
equal to the mass of the object. 

     In addition, Archimedes' Principle can be demonstrated by having students 
construct a small barge with aluminum foil with measurements about 10 square 
centimeters.  Have students get as many pennies as possible on their barges.
Afterwards, one should explain Archimedes' Principle in relation to why ships 
float.