Density

Marilyn Martello               Clinton
                               6110 N Fairfield
                               Chicago IL 60659
                               (312) 534-2025

Objectives:
     
     Grade 6-8
     To discover what properties determine the phenomenon of objects able to 
      float and objects able to sink.
     To identify these properties as mass and volume.
     To apply density to the three different types of matter:  solid, liquid,  
      and gas.  
                                 
Materials Needed:

   For 4 groups of 4 students:
   8 objects that will float, 8 objects that will sink (it is important that 
   the objects chosen should differ greatly in density so that when you plot 
   your graph, the objects which float should be way below water density and the 
   objects which sank should be way above the density of water ) 
   4 plastic pails filled 3/4 with water
   2 scales with measurements in grams
   4 beakers with these measurements: 200ml, 500ml, 1000ml, 2000ml
   3 graduated cylinders measuring 25ml, 100ml, 1000ml
   4 oz cooking oil, 4 blue colored ice cubes 
   12 oz clear glass

Strategy:

   In the 12 oz glass, put in oil and ice cubes and set aside.  Choose an object 
which will float and one which will sink.  Tell the class you have two objects 
you want them to observe.  Drop them into a clear beaker filled 3/4 way with 
water and ask what happened.  Students will suggest that weight is a factor that 
made one object sink.  At this point instruct the class to divide themselves 
into groups of 4, come and choose an object of their choice, weigh their object, 
put the object in water, and record their findings on a chart on the board as 
follow: 

    _________________________________________________________________
    |  Name           |   Object      |   weight   | sink or float   |
    |_________________|_______________|____________|_________________|
    |                 |               |            |                 |
    |                 |               |            |                 |
    |

After everyone had recorded their findings, have the class look at the chart to 
find any type of pattern (there won't be any).  Guide the class to discover that 
volume of an object is another factor that needs to be explored.  Show how to 
measure the volume of an object by displacement of water using a graduated 
cylinder.  Drop an object into the cylinder filled 3/4 way with water.  The 
water level will rise.  Tell the class that they are to mark the water level 
before and after the object has been dropped.  Take the difference between the 
two markings and record it on the column marked volume on the board (add a 
volume column to the chart).  When everyone has completed their data, explain 
to the class that they have found the density of their object.  Write the 
formula on the board: Density = Mass/Volume or D = M/V.  Using the data from 
the chart, construct a graph plotting the volume vertically and the mass or 
weight horizontally.  Ask if anyone sees a pattern as to why some objects 
floated while others sank (again there is none).  Point to the fact that we need 
to plot the density of water on the graph also because the objects were floating 
in that medium.  Help the class measure and weigh water using different amounts 
3 times and plot the data on the graph.  Draw a line connecting the water "dots" 
on the graph.  Ask the class to look at the graph again.  Using the information  
on the graph, ask if anyone can now explain why some objects floated and why 
others sank.  If no one has the correct explanation, circle the "dots" of the 
objects which floated (they should all be above the water line).  Point to the 
fact that all the floating objects are above the water line and the  ones which 
sank are below the line.  In other words, objects less dense than water will 
float and objects more dense than water will sink.  Apply this concept to gases.  
Ask the class why does a balloon filled with helium floats in air (helium is 
less dense than air therefore it rises). 

Performance Assessment: 

Show the class the 12 oz glass containing the oil and ice cubes (the ice should 
have melted enough to form a blue fluid accumulation at the bottom of the 
glass). 
Ask:  Which fluid is more dense, the oil or the blue water?