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ACTIVITY OVERVIEW

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Properties of Plastics

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In this activity students determine the chemical and physical properties of four plastics. They then test a “mystery” sample to find out what it is. Based on the plastics’ chemical and physical properties, students relate possible uses for each plastic to its properties.

KEY CONCEPTS AND PROCESS SKILLS (with correlation to NSE 5–8 Content Standards) 1.

Plastics can be identified based on their physical properties, including flexibility, hardness, and density. (PhysSci: 1)

2.

The properties of a material determine how it can be used. (PhysSci: 1)

3.

Making decisions about complex issues often involves trade-offs. (Perspectives: 4)

4.

Scientists think critically and logically to make the relationship between evidence and explanation. (Inquiry: 1)

KEY VOCABULARY chemical property physical property plastic relative density

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Activity 18 • Properties of Plastics

MATERIALS AND ADVANCE PREPARATION For the teacher 32 vials with caps 8

packets of table salt ethanol

*

water 10-mL graduated cylinder 2

strips each of: blue polypropylene (PP) green polyvinyl chloride (PVC) red high-density polyethylene (HDPE) yellow polystyrene (PS)

8

strips each of four unknown plastics (clear, black, orange, white)

1

bottle of acetone

*

1

glass container (beaker, petri dish, or jar)

*

1

heat lamp, hairdryer, or other heat source

*

1

metal ring stand and clamp (or similar setup)

*

2

wood strips or glass slides

*

1

pair of heavy-duty scissors (optional)

*

1

overhead projector

*

paper towels 1

Transparency 18.1, ”Four Common Plastics”

1

Transparency 18.2, “Some Common Plastics and Synthetic Fibers

1

Transparency 18.3, “2005 U.S. Plastics Production”

1

Transparency 18.4, “Plastic Recycling Codes” Scoring Guide: ANALYZING DATA (AD) and/or Scoring Guide: EVIDENCE AND TRADE-OFFS (ET)

For each group of four students *

1

petri dish or other container to hold set of plastic strips and plastic squares

1

strip and square each of: blue polypropylene (PP) green polyvinyl chloride (PVC) red high-density polyethylene (HDPE) yellow polystyrene (PS) unknown plastic (orange, clear, black, or white)

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1

30-mL graduated cup

1

vial of alcohol

1

vial of water

1

vial of alcohol-water mixture

1

vial of saltwater

Properties of Plastics • Activity 18

*

1

15-mL bottle of hydrochloric acid

1

paper clip

1

pair of plastic forceps

1

additional plastic strip from plastic containers brought from home (optional)

For each student *

1

Student Sheet 18.1, “Properties of Different Plastics” (optional)

1

pair of safety goggles

1

copy of Scoring Guide: ANALYZING DATA (AD) (optional) and/or

1

copy of Scoring Guide: EVIDENCE AND TRADE-OFFS (ET) (optional) *Not supplied in kit

Masters for Scoring Guides are in Teacher Resources III: Assessment. For each group, prepare a set of four labeled vials, each containing one of the following solutions: alcohol, water, alcohol-water, and saltwater. Alcohol: fill with 8mL of denatured alcohol (ethanol) Alcohol–water: fill with 4mL of alcohol and 4 mL of water Water: fill with 8mL of water Saltwater: fill with 8mL of water and one packet of salt. Place the blue, green, red, and yellow plastic strips and squares for each group in a small container for easy distribution. You will distribute the orange, clear, black, and white plastic pieces when the students begin Part B of the Procedure. Be prepared to provide fresh strips for each group. Set up the equipment for the effects-of-heat test and effects-of-acetone test before class. You may want to start these demonstrations as described in Teaching Suggestion 2 that there will be observable differences in the plastic pieces to show the class in Teaching Suggestion 3. To demonstrate the effects-of-heat test you will need to set up a ring stand with a heat source attached to it. Use a heat-lamp apparatus or a hair dryer as the heat source. Clamp the four plastic strips to the ring stand, and with two strips of wood or two glass slides sandwich the ends of the strips flat in the ring stand. A diagram of the setup is shown below.

B-67 2461 LabAids SEPUP IAPS TG Figure: PhysTG B 18.01

Activity 18 • Properties of Plastics

Students can test additional plastics by bringing in clean, empty plastic containers from home. You can then use scissors to cut plastic strips for testing. Because of the shape and sturdiness of many plastic containers, cutting strips can be time-consuming and possibly hazardous were students to do the cutting. If you are planning to have students test additional strips, be sure to prepare them in advance. Containers for shampoos, medicines, milk, soft drinks, juices, or other foods are best. Be sure to eliminate the recycling code on the containers from the samples. The recycling code shows a number that identifies the type of plastic, as shown on Transparency 18.4, “Plastic Recycling Codes.”

SAFETY Students should wear safety glasses during this investigation. It is recommended that students wear protective plastic gloves when working with caustics, such as hydrochloric acid. Note, however, that the concentration that is used in this activity is low enough that wearing gloves is not imperative. Check your school’s and district’s safety requirements to determine if you must provide them in this activity. Acetone is an extremely flammable liquid. Keep acetone and its bottle away from heat, sparks, and open flames. Acetone will degrade most plastic containers except polypropylene and polyethylene. Use acetone with adequate ventilation. Avoid inhalation, ingestion, or repeated skin contact.

TEACHING SUMMARY Getting Started 1.

Discuss the useful properties of plastic.

Doing the Activity 2.

Students investigate the properties of four plastics.

3.

Students test the chemical and physical properties of an unknown plastic .

Follow-Up 4.

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(AD ASSESSMENT) The class discusses how the properties of plastics affect their use and applies evidence to identify the unknown plastic sample.

Properties of Plastics • Activity 18

BACKGROUND INFORMATION Plastics The first completely synthetic plastics to be commercially produced were those made from phenol and formaldehyde. Leo Baekeland developed the chemical process in 1907 which created “Bakelite,” the first synthetic plastic. Phenolic plastics eventually led to the development of urea and thiourea, which were used to make such products as foam rubber cushions and melamine laminate furniture. As petroleum became a major source of polymers the development of plastics through the 20th century accelerated and organic chemists invented sophisticated techniques that allowed them to tailor a plastic for a specific purpose. Today, most synthetic plastics are produced from oil or natural gas. Crude oil is composed of hydrocarbons that vary in length and configuration and, as a result, have different properties. To separate these hydrocarbons, chemists heat crude oil to 400˚C. Because the various hydrocarbons have different boiling points, they condense at different temperatures. For example, naptha, a liquid, is made up mostly of molecules containing 8–12 carbon atoms, and it condenses at 150˚C. Gases, including ethane, methane, and propane, generally have 1–4 carbon atoms and condense at 110˚C. The smaller hydrocarbons, particularly ethane (which is converted into ethylene), are used for the manufacture of a whole variety of polymers, which will be treated in more depth in later activities. About 84% of a typical barrel of crude oil is converted to fuels that are burned for heating or to gasoline and jet fuel. Less than 3% of the oil goes into the synthesis of plastics. Even though the manufacture of most plastics begins with just carbon and hydrogen, other elements can be involved. Oxygen, chlorine, fluorine, nitrogen, silicon, phosphorous, and sulfur are added to hydrocarbon chains to create certain plastics. Polyethylene (PET) contains only hydrogen and carbon, while tetrafluoroethylene, better known as Teflon, contains fluorine in place of the hydrogen atoms. There are two categories of plastics: 1. Thermoplastics are those that can be continually and repeatedly formed and reshaped with the application of heat and pressure. About 85% of all plastics produced are thermoplastics. Most recyclable plastics, such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride are thermoplastics. 2. Thermosets are those that cannot be reshaped once they are formed. This is usually because those plastics have been cross-linked, and the cross-linked bonds cannot be broken. About 15% of all plastics produced are thermosets. Examples include polyester, polyurethane, and epoxy resins such as silicone.

REFERENCES American Plastics Council. PIPS Year-end Statistics for 2005. Production, Sales, and Captive Use. Plastic Industry Producers’ Statistics Group. Retrieved January 2007 from www.americanplasticscouncil.org/s_apc/docs/1700/1678.pdf.

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Activity 18 • Properties of Plastics

TEACHING SUGGESTIONS GETTING STARTED 1.

Discuss the useful properties of plastic.

Ask each student to name a product made of plastic. Next ask, What makes plastic a useful material? Develop a list of students’ responses. Some possible answers are that plastics are lightweight, strong, long lasting (durable), flexible, and available in all sorts of shapes and colors. Point out that often one plastic item has properties that are not shared by another plastic item. For example, the clear plastic wrapping on items such as CDs is easy to tear, while plastic detergent bottles are fairly sturdy. This suggests that there are different types of plastic, each with its own physical properties. Emphasize that physical properties of a substance are characteristics that can be observed without seeing how it reacts chemically with other substances. Make a list of some physical properties that might distinguish plastics from other materials. Prompt students to think back to the materials they considered for drink containers in Activity 12, “Evaluating Materials,” and Activity 13, “ Life Cycle of a Material.” Ask, If you were going to manufacture a product such as a drink container, why might you make it out of plastic? To this students may respond that containers made of plastic are strong, durable, flexible, and lightweight. They might also add that it comes in many colors. Then ask, What are some physical properties of plastics? Possible responses include that they are hard, solid, flexible, and light in weight. Point out that they are in fact relatively low in density (mass per volume) compared to glass and most metals. If students suggest color, point out that it is not a physical property of the material itself. Reiterate that most things made of plastic can be dyed any color. Because of this, color is not an inherent physical property for characterizing plastics. In this activity, students will identify the properties of four different types of plastics.

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DOING THE ACTIVIT Y 2.

Students investigate the properties of four plastics

Tell students they will investigate some of the chemical and physical properties of four plastics. They will use these characteristics to distinguish one type of plastic from another and then use their data to identify an unknown, or “mystery,” plastic. Explain to students that their investigations involve a variety of chemicals and equipment. Review classroom safety guidelines. Go over how to pronounce each of the four types of plastics. Display Transparency 18.1, “Four Common Plastics.” Explain that the four pieces they will test fall into one of these four types. There are many more types, but these are the four major types they will investigate. Point out the color coding that has been done so students visually can tell one piece from the next. Review the physical and chemical properties listed in Table 1, “Testing Properties of Plastics,” in the Student Book. Explain to students that since plastics can be molded and dyed, the shape and color of a plastic is not characteristic of the type of plastic. Review the information in the table that describes how to test the properties of the plastic samples. This activity provides the opportunity for students to construct their own data table. You may decide as a class or in groups of four the best data table format, which will reinforce students’ ability to design a data table. Depending on the developmental appropriateness, however, you may choose for students to use optional Student Sheet 18.1, “Properties of Different Plastics,” to record the results of their tests. As necessary, review the tests that students will conduct. One additional test, for which the materials have been supplied, but that is not described in the student book, is the affect of hydrochloric acid. To conduct this test, show students how to drop 2 drops of the acid on each of the strips of plastic. After observing if the acid and the plastic react, students should dip the plastic in water to rinse it, and dry it with a paper towel.

Properties of Plastics • Activity 18

While students are completing their tests, demonstrate for the class the effects of acetone and heat on the four known plastics as described in the Materials and Advance Preparation section. Students should notice that polystyrene bends the most in response to heat, followed by polyvinyl chloride. Polypropylene and high-density polyethylene are unaffected. Polystyrene is soluble in acetone and may swell and become very loose and stretchy. The polyvinyl chloride will react to acetone in the same way but to a lesser degree. Sample results for all tests are shown in the table below. Testing the Effect of Heat on Different Plastics Clamp the four plastic strips to the ring stand as shown below. Use two strips of wood (or two glass slides) to sandwich the ends of the strips. Use a heat lamp or hair dryer to test the effect of heat on the plastics. Turn the heat source on the samples for at least 10 minutes, making sure that the heat falls evenly on all four strips. You may begin the test while students are completing their own investiga-

tions, or you may have them observe the entire test as you conduct it. If you use a heat lamp (or 150–200 watt light bulb) with a reflector as your heat source, the reflector must have a ceramic base. Be careful when handling these items; they can become very hot. An illustration of the setup is shown in Advanced Preparation above. Testing the Effect of Acetone on Different Plastics Pour a small amount of acetone into a glass container, using enough to cover half the length of a plastic strip. (Because acetone quickly evaporates and diffuses through a room, you should wait to pour the acetone until right before you conduct the acetone test, and cover the container while conducting the test.) Place one strip of each of the four plastics in the acetone for at least five minutes. After five minutes, remove the strips from the acetone and place them on a paper towel in a well ventilated area away from any flames or sources of combustion. Students can then examine the strips.

Sample Results, Student Sheet 18.1, “Properties of Different Plastics” Polypropylene (PP)

Polyvinyl chloride (PVC)

High-density polyethylene (HDPE)

Polystyrene (PS)

Plastic is color-coded

Blue

Green

Red

Yellow

Flexibility

Fairly flexible

Very flexible

Fairly flexible

Not very flexible

Crease color

Slightly white

Still green

Still red

White

Hardness

Gets scratched

Gets scratched

Gets scratched

Gets scratched

Density relative to alcohol

Sinks

Sinks

Sinks

Sinks

Density relative to alcoholwater mixture

Floats

Sinks

Sinks

Sinks

Density relative to water

Floats

Sinks

Floats

Sinks

Density relative to saltwater

Floats

Sinks

Floats

Floats

Effect of acetone

None

Softens

None

Softens greatly

Effect of heat

None

Bends

None

Bends greatly

Reaction with HCl

None

None

None

None

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Activity 18 • Properties of Plastics

3. Students test the chemical and physical properties of an unknown plastic. Tell students that they will now receive samples of a mystery plastic. This plastic has been dyed a color different than the four original samples. Not all groups will be receiving samples of the same color. Ask the student groups to decide which of the tests used in this investigation they will conduct to identify the plastic, as described in Procedure Step 4. Then they should design a data table to record their results. After students have constructed their data tables, give each group a sample of one of the unknown plastics, and allow them to conduct their tests. They should check their results with the information they recorded for each strip in their data table, “Properties of Plastics,” to determine the type of the mystery plastic. If they do not have adequate information, encourage groups to conduct additional tests until they are able to identify the mystery plastic. The color key that follows will help you check the results of students’ tests. The results for each plastic are shown in the table “Properties of Different Plastics” in Teaching Step 2. Students are not given this color key. Key to Plastic Types Plastic Type

Color of identified plastic strip

Color of “unknown” plastic strip

Polypropylene (PP)

blue

clear

Polyvinyl chloride (PVC)

green

orange

High-density polyethylene (HDPE)

red

black

Polystyrene (PS)

yellow

white

FOLLOW-UP 4.

(AD ASSESSMENT) The class discusses how the properties of plastics affect their use and applies evidence to identify the unknown plastic sample.

Ask students to identify the similarities and differences among the plastics they tested. Some of the similarities, which may not be obvious to students, are shown in their results in their tables, “Properties of Different Plastics.” The plastics tested are fairly low in density, somewhat flexible, scratch easily, and are easily molded and dyed. In Analysis Questions 1 and 2 students are asked to identify specific properties and to back their answers with their test results. Summarize the activity by highlighting the fact that plastics are materials that can be shaped. The word plastic itself comes from the Greek word plastikos that refers to the ability to be molded or shaped. Suggest the following as a working definition for plastics: Plastics are a group of materials manufactured primarily from petroleum and natural gas. Many plastics are used to create products because they can easily be formed, dyed, and molded into any shape. Begin by asking students, Which of the four types of plastic you tested do you think is used most today and why? Students may postulate that polystyrene is used extensively because of its relative inflexibility, which could make it more sturdy and good for forming food containers. They might also say that low-density polyethylene would be used a lot to hold certain chemical substances since it does not weaken in acetone. To give students an idea of the myriad types and names of common plastics and those that are produced in the greatest numbers, project Transparency 18.2, “Some Common Plastics and Synthetic Fibers,” and Transparency 18.3, “2005 U.S. Plastics Production.” You may choose to ask the class to summarize the information shown on Transparency 18.3 as an exercise in interpreting data contained in a table. Point out that it is not usually easy to tell one plastic from another by simply looking. One way plastic

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Properties of Plastics • Activity 18

manufacturers indicate the type of plastic is imprinting its recycling code somewhere on the product. Project transparency 18.4, “Plastic Recycling Codes,” to show students the codes for types of plastic containers. By finding this number on the container, they will be able to easily identify the type of plastic it is made of. For Analysis Question 3, students’ responses will depend on which plastic sample that they are given to identify. They should support their identification with their test results and then compare the physical properties of the unknown with the four known plastic types investigated in Part A. Check their responses for consistency between the properties of the plastic and its identification. Use the ANALYZING DATA (AD) Scoring Guide to provide feedback on students answers. A sample Level 3 response is provided below.

SUGGESTED ANSWERS TO QUESTIONS 1 Using evidence from your table, “Properties of Plastics,” identify the plastics which were scratched. How will this property affect their use? All of the plastics were scratched. 2. Below is a scale showing the relative density of the four liquids you used to determine the density of the plastics. a. Which plastic sample was the most dense? Explain your evidence. Polyvinylchloride (PVC) is the densest because it sinks in saltwater, the densest solution. The others float in saltwater. b. Which plastic sample is the least dense? Explain your evidence. Polypropylene is the least dense because it floats in the alcohol/water mixture, and all of the other samples sink.

Analysis Question 4 serves as a review of the concept of relative density first developed in Issues and Physical Science, Unit A, Activity 7, “Properties of Solids.” You might find it useful to put one of each of the plastic squares in each of the liquids to provide a visual as students answer this question. Then lining up the liquids as they are shown in the diagram will help students determine which liquid is the most dense and which it is the least dense.

c. In your science notebook, make a copy of the scale shown above. On it draw a labeled arrow that shows where each plastic fits on this scale of relative density. Relative density

0.80 Alcohol

EXTENSION

0.90 Alcohol/water PP

To further explore and identify kinds of plastics you can have students test samples they bring from home. Some students may be frustrated to find that some of the samples do not exactly fit the properties found with the pure sample strips from the kit. Tell them that plastic products often contain additives, such as fillers, plasticizers (which make them more flexible), stabilizers, and lubricants that may alter the properties. If students cannot make an exact match from their data, they should select the plastic that their sample resembles most closely.

3.

HDPE

1.10 Saltwater

1.00 Water PS

PVC

2561 LabAids SEPUP IAPS SB ASSESSMENT ) What type of plastic is your (AD Figure: PhysTG B 18.02 LegacySansMedium 10/11.5 mystery sample? Explain the evidence you used to

come to this conclusion. Level 3 Response The white unknown sample is polystyrene. It is not very flexible, the crease color is white, and it sinks in water and water-alcohol mixture, but floats in saltwater. Further testing showed that the unknown plastic piece is softened greatly by acetone and bends greatly when heat is applied. It was also observed that it gets scratched, but all of the plastics shown in the table do, so this test doesn’t help determine which type of plastic it is.

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Activity 18 • Properties of Plastics

4.

You are a materials scientist asked to recommend a type of plastic for manufacturing three products. Using the information in your table, “Properties of Plastics,” choose the best plastic for each of the following products. Be sure to include evidence from the activity in your answer. a. nail polish (high in acetone)container Polypropylene or high-density polyethylene because they are not affected in acetone since it needs to not soften when containing acetone. b. dishwasher-safe food container Polypropylene or high-density polyethylene because they are not affected by heat, and a dishwasher-safe container needs to be able to withstand high temperatures. c. sports drink container High-density polyethylene because it does not crease, and is not affected by heat. So if the drink bottle sits in the sun it will not be affected.

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Four Common Plastics

Type of Plastic

Common Uses

Polypropylene (PP)

Ketchup bottles, yogurt containers

Polyvinyl chloride (PVC)

Shampoo and salad dressing bottles

High-density polyethylene (HDPE)

Milk containers, orange juice bottles, and bleach bottles

Polystyrene (PS)

Prescription bottles, plastic utensils, cups

©2007 The Regents of the University of California

Color Key Polypropylene (PP)

Blue

Polyvinyl chloride (PVC)

Green

High-density polyethylene (HDPE)

Red

Polystyrene (PS)

Yellow

Issues and Physical Science • Transparency 18.1

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Some Common Plastics and Synthetic Fibers

©2007 The Regents of the University of California

Material name

Common or trade names

Some uses

Acrylic

Acrylan, Orlon

Sweaters, carpets

Cellulose acetate

Tenite, Chromspun, Celera

Toys, plastic forks, double knit fabrics, curtains

Nylon

Cantrece, Antron

Clothing, carpets

Polyacrylic acid

Acrylic paint

Cards, homes, art work

Polyacrylonitrile

Orlon, Acrilan

Clothing fabrics

Polybutadiene

Rubber

Automotive tires

Polycarbonate

Lexan, Merlon

Football helmets

Polyethylene

Alathon

Shopping bags, electrical insulation

Polyethylene terephthalate (polyester)

Mylar, Dacron, Avisco, Jetspun, Zantrel

Soft drink bottles, photographic film, audio tapes, clothing, fabric

Polymethacrylate

Lucite, Plexiglass

Aircraft windshields and windows

Polypropylene

Herculon, Vectra

Luggage, fabrics

Polystyrene

Styrofoam

Foam cup

Polytetrafluoroethylene

Teflon

Stain-proof coating on upholstery, non-stick coating on cookware

Polyurethane

Foam rubber

Sofa cushions

Polyvinyl acetate

Vinylite

Chewing gum, adhesives

Polyviny chloride

Naugahyde, Koroseal

Raincoats, drain pipes, records

Silicone

RTV 615, Silastic

Water-repellant coatings, lubricants

Spandex

Lycra, Spandelle

Elastic waistbands, tights, ski pants

Viscose rayon

Cellophane

Transparent tape

Issues and Physical Science • Transparency 18.2

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2005 U.S. Plastics Production

©2007 The Regents of the University of California

Type of Plastic

Abbreviation

Common Uses

Percentage of U.S. Plastics Production in 2005 21%

Polypropylene

PP

Drinking straws, bottle caps and lids

Polyvinyl chloride

PVC

Food wraps, water pipes

17%

High-density polyethylene

HDPE

Milk jugs, detergent bottles

19%

Liner low-density polyethylene

LLDPE

Low-density polyethylene

LDPE

Grocery bags, trash bags

9%

PET, PETE

Soft drink bottles, water bottles

9%

Polystyrene

PS

Fast food containers, foam cups

7%

Other plastics

—

Packaging

3%

Nylon

—

Clothing

1%

Thermoplastic polyesters (primarily polyethylene terephthalate)

Issues and Physical Science • Transparency 18.3

14%

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Plastic Recycling Codes Plastic Code

Abbreviation

Common Uses

PET, PETE

Polyethylene, terephthalate

Soft drink bottles, water bottles

HDPE

High-density polyethylene

Milk jugs, detergent bottles, water bottles

PVC

Polyvinyl chloride

Food wraps, water pipes, shampoo bottles

LDPE

Low-density polyethylene

Grocery bags, trash bags

Polypropylene

Drinking straws, bottle caps and lids

PS

Polystyrene

CD jackets, fast-food containers

—

Other plastics

Packaging, some food bottles

PP

©2007 The Regents of the University of California

Type of Plastic

Issues and Physical Science • Transparency 18.4

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Name

Date

Properties of Different Plastics

Plastic is color-coded

Polypropylene (PP)

High-density Polyvinyl Chloride (PVC)

Polyethylene (HDPE)

Polystyrene (PS)

Blue

Green

Red

Yellow

Unknown plastic

Flexibility

Crease color

Hardness Density relative to water Density relative to saltwater

©2007 The Regents of the University of California

Density relative to water-alcohol mixture Density relative to alcohol Reactivity with hydrochloric acid Effect of acetone Effect of heat

Issues and Physical Science • Student Sheet 18.1

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