Using Density to Identify Metals

Elaine Hunter                  Zenos Colman School
                               4655 South Dearborn Street
                               Chicago, Illinois 60609
                               312-536-5500

Objective:

     (Recommended grade levels 6-8)

     To teach the techniques of measuring the density of some common metals.
                                      
     To identify a metal by its density.

Materials:

     graduated cylinders (10, 25, 50, and 100 ml)/group
     electronic balance (one for the class)
     calculators (one for each group)
     unidentified metals (iron, copper, aluminum, lead, pyrite)
     water bottles
     distilled water
     thread
     cotton balls
     medium size rock
     two pan balance
     lab worksheet to record mass, volume and density

Strategy:

Prior to teaching the lesson ask the class to bring in some gold jewelery that 
they would like to have appraised.  The day of the lesson write the following 
riddle on the board:  Which weighs more, a pound of feathers or a pound of 
rocks?  This riddle should be solved before the lesson begins.  Discuss the 
solutions to the riddle with the class.  Show the class a medium size rock, and 
ask a volunteer to balance the rock with cotton balls using a two pan balance. 
Discuss the results with the class.  Elicit reasons why so many cotton balls 
were needed to balance the medium size rock.  Discuss the meaning of density as 
a ratio of mass to volume.  Tell the class that density can be expressed 
mathematically: D=M/V.  Practice calculating a few density problems.  Density is 
like a finger print for metals.  If the density of a metal is known, it is a 
good clue to the identity of the metal.  Give the class a handout that explains 
why prospectors during the goldrush in 1848 experienced difficulty in finding 
gold.  Ask the class what was mistakenly found.  How can we distinguish "Fool's 
Gold" from pure gold?  Let the class examine samples of "Fool's Gold" and 
discuss properties of the sample that might have led prospectors to believe they 
had pure gold. 
 
Tell the story of an envious king who was duped by some crafty crooks who knew 
how to make some inferior metals look like gold, silver, or platinum.  In the 
lab the class will perform experiments with metals to determine which metals 
were subtituted for the metals used in the king's crown, scepter, breast plate 
and sword.  They will do this by measuring the mass and volume and calculating 
the density. 


Conclusions:

The class will average the density results and make a standard for metals.
The class will compare their results with those of experts.
The class will determine which metals were used as substitutes for the king's 
  metals.  
The class will use the data chart to compare the mass and volume to the metal 
  having the greatest density with that of the lowest density, and see if there 
  is a relationship. 

Evaluation:

Each student should have completed his lab sheet and have calculations for 
  densities of the metals.  
Each student should have performed each step in finding density in the lab.
The teacher should observe involvement of the students during class discussions.

References:

Marshall, Robert H. and Jacobs, Donald H. Physical Science Investigating Matter
and Energy Media Materials, Inc., Baltimore, Maryland, l987

Bernstein, Leonard; Schacter, Martin; Winkler, Alan; Wolfe, Stanley. Concepts
and Challenges in Science Book 2, Cebco Standard Publishing, Fairfield, New
Jersey