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Identifying Plastics with Density Data (2 Favorites)

ACTIVITY in Separating Mixtures, Density, Polymers, Polymers. Last updated September 29, 2020.


Summary

In this activity, students will familiarize themselves with different types of plastics. Using data analysis, students will determine how to use the density values of a variety of plastic samples in order to separate a specific sample from a mixture.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

  • HS-ETS1-2: Design a solution to a complex real-world problem by breaking it down into small, more manageable problems that can be solved through engineering.
  • HS-ETS1-3: Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.
  • Scientific and Engineering Practices:
    • Analyzing and Interpreting Data

Objectives

By the end of this activity, students should be able to

  • Understand how differing densities of liquids and solids allow for mixtures to be separated.

Chemistry Topics

This activity supports students’ understanding of

  • Density
  • Separating mixtures
  • Polymers

Time

Teacher Preparation: minimal
Lesson: 20 minutes

Materials

  • Student Handout

Safety

  • No specific safety precautions need to be observed for this activity.

Teacher Notes

  • Teachers may want to use the lab, The Big Six Plastics, after completing this activity.
  • This “Your Turn” is an activity from Chemistry in Context, 10e that you can use to warm students up before conducting the lab portion of the lesson. Your Turn activities are sprinkled throughout the book to give students an opportunity to practice skills and test their understanding. This particular type of “Your Turn” activity is “Concentrating on Concepts.” Often times these types of activities involve working with data and applying them to a real-life scenario.

  • Related background information for teachers/students:
    • According to the U.S. Environmental Protection Agency, in 2017, the U.S. generated 258 million tons of municipal solid waste. It contained tens of millions tons of plastics, including over 12 million tons of containers and packaging. Of all these plastics, only 9% is recycled. This rate is well below those for other materials such as newspapers (67%), aluminum cans (55%), and glass containers (34%). If you include other types of waste, including industrial, medical, hazardous, and agricultural waste, this amount of waste generated by the U.S. in 2017 was 8.4 billion tons.
    • The majority of plastic waste is composed of six common polymers, “The Big Six.” See Section 9.1 in Chemistry in Context, 10e, for more information about these polymers.
    • Compliance in labeling plastic is voluntary, and not all plastics have an identification code symbol. Without code numbers, plastics are difficult to separate by appearance.

For The Student

Instructions

The table below contains the density values for PET and for three other plastics likely to be found with it in a recycling bin:

Plastic
Density (g/cm3)
PET
1.38 – 1.39
HDPE
0.95 – 0.97
PP
0.90 – 0.91
PVC
1.30 – 1.34

When dropped into a liquid, a plastic will float or sink depending on the density of the liquid. Here are the densities for several liquids that do not degrade the four plastics above:

Liquid
Density (g/cm3)
Methanol
0.79
42% Ethanol/Water mixture
0.92
38% Ethanol/Water mixture
0.94
Water
1.00
Saturated solution of MgCl2
1.34
Saturated solution of ZnCl2
2.01

Task

Given a PET sample contaminated with HDPE, PP, and PVC, propose a way to separate the PET from the other three plastics. Assume that all density values were measured at the same temperature.