Foundations of Matter and Chemical Change · 5th Year · Stoichiometry and the Mole Concept · Summer Term

Investigating Plastics

Explore different types of plastics, their properties, and the challenges and benefits of their widespread use.

• NCCA Curriculum Specifications
• NCCA: Primary - Materials - Properties and Characteristics
• NCCA: Primary - Environmental Awareness and Care - Waste Management

About This Topic

Plastics represent a diverse group of synthetic polymers formed by linking monomer units, often derived from petroleum-based hydrocarbons through polymerization reactions. Students classify common types such as polyethylene (flexible packaging), polystyrene (foam insulation), and PVC (pipes), testing properties like density, elasticity, melting point, and solubility in solvents. These investigations tie directly to stoichiometry as students calculate molar masses of repeat units and predict material behaviors from molecular formulas.

The topic examines plastics' ubiquity due to low production costs, durability, and versatility in everyday items from bottles to medical devices. Students weigh these benefits against environmental challenges: slow degradation causing microplastic pollution, reliance on finite fossil fuels, and recycling inefficiencies from mixed waste streams. This analysis builds skills in evidence-based evaluation and connects to broader chemical change principles.

Hands-on approaches excel for plastics because students handle real samples to test properties, conduct waste audits, and prototype recycled designs. These methods make molecular concepts visible, encourage data-driven discussions, and cultivate awareness of sustainable practices.

Key Questions

1. What are plastics made of?
2. Why do we use so much plastic?
3. What are the problems with plastic waste?

Learning Objectives

• Classify common plastics into categories based on their chemical structure and observable properties.
• Analyze the relationship between a plastic's molecular structure and its physical characteristics, such as elasticity and melting point.
• Evaluate the environmental impact of different plastic types, considering factors like biodegradability and recyclability.
• Calculate the molar mass of the repeating unit for common polymers like polyethylene and polystyrene.
• Design a simple experiment to compare the solubility of different plastics in common solvents.

Before You Start

Introduction to Polymers

Why: Students need a basic understanding of what polymers are and that they are made of repeating units before investigating specific types of plastics.

Physical Properties of Matter

Why: Understanding concepts like density, solubility, and melting point is essential for testing and comparing the characteristics of different plastics.

Key Vocabulary

Polymer	A large molecule made up of many repeating smaller units called monomers. Plastics are synthetic polymers.
Monomer	The small, repeating molecular unit that links together to form a polymer. For example, ethylene is the monomer for polyethylene.
Polymerization	The chemical process where monomers join together to form a long polymer chain.
Biodegradability	The ability of a material to break down naturally through the action of microorganisms over time. Many plastics are not readily biodegradable.
Recycling Codes	Numbers within a chasing arrows symbol used to identify different types of plastic resins, aiding in sorting for recycling.

Watch Out for These Misconceptions

Common Misconception

All plastics behave the same way when heated.

What to Teach Instead

Thermoplastics soften and remold while thermosets char without melting due to cross-linking. Hands-on burn tests with safe samples let students observe differences firsthand, prompting them to revise models through group comparisons.

Common Misconception

Plastics biodegrade like food waste.

What to Teach Instead

Plastics persist for centuries, fragmenting into microplastics rather than breaking down. Burial simulations or long-term exposure logs in class reveal this, with discussions helping students connect persistence to pollution impacts.

Common Misconception

Recycling eliminates all plastic waste problems.

What to Teach Instead

Contamination and sorting errors limit recycling rates to under 10% globally. Waste sorting activities expose these issues, as students tally rejects, fostering realistic views on sustainable solutions.

Active Learning Ideas

Stations Rotation: Plastic Property Tests

Prepare stations with plastic samples (PET, HDPE, PP). Include float/sink for density, bend tests for flexibility, and hot pin for burn characteristics. Groups rotate every 10 minutes, tabulate results, and identify plastics by code. Conclude with class chart comparing properties.

45 min·Small Groups

Whole Class: School Waste Audit

Collect one week's plastic waste from classrooms. Sort into types, weigh portions, and calculate percentages by category. Discuss sources and recycling feasibility, graphing data to visualize waste patterns.

40 min·Whole Class

Pairs: Polymer Chain Models

Use pipe cleaners or straws as monomers to build linear and branched polymer models. Test model flexibility to link structure to properties. Pairs present how changes in repeat units alter real plastic behaviors.

25 min·Pairs

Small Groups: Recycling Challenge Design

Provide mixed plastic scraps. Groups design and prototype a useful item like a planter, considering sorting and joining methods. Share prototypes and evaluate feasibility for waste reduction.

50 min·Small Groups

Real-World Connections

• Materials scientists at companies like Dow or DuPont develop new plastic formulations for specific applications, such as lightweight components for electric vehicles or durable packaging for food preservation.
• Environmental engineers working for local councils or waste management firms conduct waste audits to assess the types and quantities of plastics entering landfills and recycling facilities, informing policy decisions on waste reduction.

Assessment Ideas

Quick Check

Provide students with samples of 3-4 common plastics (e.g., PET bottle, HDPE milk jug, LDPE plastic bag, PP container). Ask them to record observations on density (will it float in water?), flexibility, and texture, then assign each a potential recycling code based on appearance and properties.

Discussion Prompt

Pose the question: 'Given the benefits of plastics in medicine and food safety, how can we best mitigate the environmental problems caused by plastic waste?' Facilitate a class discussion where students propose solutions, referencing concepts like biodegradable alternatives, improved recycling infrastructure, and reduced single-use plastic consumption.

Exit Ticket

Ask students to write down one type of plastic they encountered today, its primary use, and one property that makes it suitable for that use. Then, have them list one environmental challenge associated with that plastic.

Frequently Asked Questions

What are plastics made of?

Plastics consist of polymers, long chains of repeating monomer units like ethylene for polyethylene. Derived mainly from crude oil via cracking and polymerization, their properties stem from chain length, branching, and additives. Students explore this through stoichiometry calculations of molar masses and simple synthesis demos linking monomers to everyday materials.

Why do we use so much plastic?

Plastics offer low cost, light weight, corrosion resistance, and easy molding for applications from packaging to electronics. Production scalability from petrochemicals keeps prices down compared to alternatives like glass or metal. Balanced lessons highlight these advantages alongside waste discussions to build informed perspectives.

How can active learning help students understand plastics?

Active methods like property testing stations and waste audits engage students directly with materials, replacing abstract lectures with tangible data. Sorting real plastics reveals classification challenges, while modeling chains connects structure to function. These collaborative tasks boost retention, critical thinking, and motivation through relevance to daily waste encounters.

What activities teach plastic waste problems?

Conduct school audits to quantify plastic volumes, simulate ocean pollution with microplastic hunts in water trays, or debate recycling myths. These reveal persistence and contamination issues. Follow with redesign challenges using scraps, encouraging innovative solutions and linking chemistry to environmental responsibility.