September/October 2017 – Vol. 30 No. 1

Density Lesson

Posted: Tuesday, September 3rd, 2013

by Jeff Orlinsky

Chemistry and physical science teachers, here is a lesson on density.  It can be used with grades 10 to 12, but with some modifications it may also work with 8th or 9th grade.  This is a modification of a lab from West Catholic High School Archdiocese of Philadelphia, with references from Kenneth E. Kolb and Doris K. Kolb, Bradley University, Peoria, IL 61625 Journal of Chemical Education.

Grades: 10 – 12

Subjects: Chemistry, Earth Sciences, Physics, Physical Science, Environmental Science

Topics: Density, polymers recycling, solution preparation.

Duration: 20 min Prep + 1 hour Activity + 1 hour Post

Setting: Classroom

Materials:

  • Appropriate glassware for mixing and sharing solutions
    • Seven (7) 250-mL beakers, graduated cylinders, stirring rod, long forceps
  • NaCl 90% Ethanol, 70% isopropanol, Karo syrup, monosodium phosphate, H2O
  • Different types of plastics (see below).
  • Hole punch

Activity:

On TV, plastics from milk, soda and other food containers are shown recycled into park benches and plastic posts. However, for some uses the recycled plastic must all be the same type of material. Many municipalities have days when they pick up scrap metal, glass and plastics and some communities are urged to put the three types of materials in separate containers, but sometimes consumers sort the objects into the wrong category of waste. So, how is the waste accurately separated for recycling? Magnets can remove some metals from a moving conveyer belt.  Other metals and glass of various colors are sorted by hand.

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And, how can the plastics be separated? Magnets cannot be used. Visual inspection, even with the aid of recycling codes, is tedious. Can density help?

This lab starts with a demonstration, allows time for students to practice with the materials and apparatus, and ends with the students designing and running an experiment to identify unknown plastics.

This table identifies different polymers and their store-bought products.

Orlinsky_Table1

Table of different Polymers identified by densities and solution preparation

Orlinsky_Table2

Teacher Demonstration:

  • Apparatus:  seven (7) 250-mL beakers, graduated cylinders, stirring rod, long forceps.
  • Prepare the seven (7) beakers with the different density solutions according to the chart above.
  • Demonstrate how samples of plastic sink or float in the appropriate liquids.
  • Discuss reasons why the plastics float or sink.

Guided Student Practice:

  • Apparatus: 250-mL beaker, forceps
  • Materials: distilled water, access to beakers previously used in demonstration, two (2) plastic samples (with coded identity) for each group
  • Procedure: (DONE IN GROUPS OF 4)
    • Assign two (2) different types of plastic to each pair in groups of four (4).
    • Examine the plastics and record its code and its appearance.
      • Sample Code __________________ Sample Code __________________
      • Appearance ___________________ Appearance ___________________
    • Using the solutions, observe if the plastic to floats or sinks.
    • Verify that the plastic you have floats or sinks in the correct solution.

Student Independent Practice/Lab:

  • Objective:  Each group must predict and design a way to identify the plastics using only the known density solutions.
  • Instructions for students: Plan how to use the apparatus and materials you have been assigned to identify the pieces of plastic.
  • Students have access to the beakers from the previous demonstration, plus distilled water and normal laboratory equipment.
  • Each team executes the identification plan for the pieces of plastic they have been given.
  • To add a challenge:  all known solutions must be in a single container and students cannot have 7 beakers to test all solutions.

Use of models:

  • Using words, pictures or a physical model, show how a municipality could separate the large volume of waste plastics it collects.

Written by Jeff Orlinsky

Jeff Orlinsky

Jeff Orlinsky teaches science at Warren High School and is a member of CSTA.

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